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I will attempt to explain in short how a computer works and perhaps identify the reasons why your machine is acting slow, beach balling etc.
How computers work in brief and what you can/not change
Central Processing Unit - (CPU) or sometimes called the "processor" is basically the brains where usually most all the calculations are performed in regards to non-graphics rendering. However CPU's from Intel and AMD are now incorporating limited graphics processing ability on the CPU itself called Integrated Graphics. CPU's on Mac's cannot be changed or upgraded. Only choice is to buy a new machine, prefeably with the most CPU muscle as possible within your budget.
Random Access Memory - (RAM) this is your CPU's (and Integrated Graphics) temporary memory while it works on things your doing with the computer. RAM is very fast, but it's also volatile, if the power goes off, so does the contents of RAM. Thus why contents of your working material are saved to the more pernament boot drive. RAM can be increased on a lot of Mac computers, more RAM usually results in more performance as less of the slower storage drive is used as extra memory (aka "Swap") Most users upgrade the RAM themselves as it's a lot less expensive than the prices Apple charges. Look in your user manual, or search online, or watch instructional video's.
Graphics Processing Unit - (GPU) is a dedicated processor and memory solely for use in graphics rendering, this is known as Dedicated Graphics or video card. They are usually much more powerful than Integrated Graphics on all newer machines unless you have a older machine with weaker Dedicated Graphics. If you want to use 3D games or other graphics intensive uses, make the machine last longer, getting one with Dedicated Graphics and just not only Integrated Graphics is the answer. GPU's cannotbe changed or upgraded, except for the MacPro, provided you can find one to upgrade with. With Mac's, it's best to buy as much GPU muscle as possible with a new machine purchase.
Video RAM - (VRAM) is your amount of video RAM available to graphics rendering purposes, it can be on the dedicated graphics itself, or a portion of the RAM the CPU uses in the case of Integrated Graphics. This cannot be changed by the user, however sometimes if more RAM is installed, the VRAM for Integrated Graphics can increase in some models of Mac's.
Boot Drive - named HD for hard drive, or the newer SSD for Solid State Drive, basically whenever you see "boot drive" "HD" "internal storage drive" or any reference to the internal storage on computers it's usually referring to the main permanent storage inside your machine which holds the operating system, programs, files etc. that gets loaded into RAM or used by the computer as needed or requested. Boot drives can be replaced from the stock 5,400 RPM to a 7,200 RPM, or even a SSD, however only in the MacPro and non-Retina MacBook Pro's by the user.
Storage Drive - like a boot drive, they don't contain a operating system and used as a permanent storage for your files, usually externally. However MacPro's can have more than one drive inside the machine which any of then can be a internal storage drive or another boot drive. Can be upgraded as it's usually a external device from a third party maker and faster external drives exists. Basically a storage drive and a boot drive are the same thing, one just has a operating system to be able to boot from. It's possible to have external boot drives, like a clone for instance.
Interfaces - this is the connections between certain devices. Could be the system or frontside BUS which connects the CPU to RAM/memory, or I/O bus to connect to other parts of the computer. The faster these are and match the RAM speed, the faster the computer tends to operate. With interfacing with other hardware, like storage or boot drives, SATA 3, 2, 1, Thunderbolt, FireWire 800, are faster than FireWire 400, USB 2,1. Older Mac's don't have USB 3, keep this in mind when buying external storage devices, you can save money buying the USB 2,1 models instead. The interfaces cannot be upgraded or changed, you can only use what your Mac provides or if you have a older Mac with a ExpressCard slot, use a adapter for that.
Operating System - (OS) is the OS X based software that runs everything pretty much, from rendering what you see on the screen to the general operation of the computer, not including programs or files made with those programs by the user. There are different operating systems and different versions of those operating systems. And sometimes newer operating systems are a lot larger or have more hardware demands than your machine can currently support. Some operating systems can't run on certain types of hardware, or isn't permitted. Can be upgraded, however there is no performance improvement guaranty. For instance 10.6 is faster than 10.5 on the same hardware, but 10.7 is a hair slower than 10.6 on the same hardware.
Firmware - are small parcels of code placed in various locations like your keyboard, battery, EFI, etc. that interact with your hardware, regardless of what operating system is booted, Windows or OS X. It's what allows OS X to be reinstalled even on a new blank hard drive on newer Mac's. Controls the boot key commands you can perform if OS X is not booting etc. It's only upgradable, and then usually only using Software Update from Apple, however there is little performance loss or gain involved. If there is a problem with upgrading firmware, it can tend to break the hardware and require a logicboard replacement, so take appropriate backup and other precautions before doing a firmware update.
Programs - are software specialized to do certain tasks. Some are bundled with every new Mac or when OS X gets installed, or purchased from a third party in installed by the user. Can be upgraded, howeverit's no guaranty newer versions will be faster than the older ones, usually it's slower as developers take advantage of newer hardware performance. One should update their programs, which is different than a upgrade as updates provide security and bug fixes.
Files - are just that, files that are created either by the user (aka "User Files"), the operating system, or programs to store information in usually a permanent fashion on the Boot Drive or as backup or extra storage space on a Storage Drive.
Users files are unique and irreplaceable unlike everything else on a computer so backup! Most commonly used backup methods
Search online and download the free MacTracker and use Apple menu > About this Mac > More information "MacBook Pro 4,2" for example to use in MacTracker to find your exact machines specifications and capability. It makes little sense to spend the money for a SSD in a machine that has only SATA 1 or 2 interfaces, a 7,200 RPM drive is a better fit and more cost effective.
10.8 users may have to set System Preferences > Security > Gatekeeper to "Anywhere" to install software from the web.
About upgrading RAM or boot drive
Only the MacPro and non-Retina MacBook Pro's far as I know can one upgrade the boot drive themselves without violating the AppleCare/warranty and then the new part is not covered. Break anything in the process and it voids your AppleCare/warranty. RAM can be increased as well in more Macs besides the MacPro and MacBook Pro under the same terms and conditions.
Disassembly video's are here, however don't go past what is covered in your Mac's manual as a user replaceable/upgrade part. If in doubt, call Apple before you violate your warranty/AppleCare. Make sure the correct sized tools are used and all proper precautions taken, less you strip screws, break things that Apple is not going to cover.
Update: Retina MacBook Pro's RAM can't be upgraded and the storage drive (SSD) usually cannot be upgraded. However OtherWorld Computing has recently created a daughter card with compatibile SSD for replacement in these machines, it will likely violate one's AppleCare/warranty as "non-Apple" user/modification.
Average boot time on Mac's
With no login items or auto-start of programs like what 10.7+ does, or any network connections, your average boot time for a lot of Mac's on a 5,400 RPM hard drive should be between 30-40 seconds. About 20 seconds or so on a SSD. This is from the "bong" to full desktop, minus any log in time.
Any longer than 1 minute, I would be concerned there is something wrong and start taking some steps to identify why it's booting slow.
Advice about third party "speed" and "repair" software
Beware of software promising to "speed up" or "repair" your Mac, especially MacKeeper!!!
Look what happened here after MacKeeper was uninstalled!
Disk Warrior in my opinion is another one that preys upon the ignorant as usually the people turning to such have a much bigger problems. The file structure of Mac's don't go broken all by themselves, something causes it to occur. Find the cause instead, usually failing sectors on hard drives.
If your doing data recovery, you might have to use Disk Warrior on a external boot drive to "fix" the internal drive before being able to transferring files. However if you made timely backups of your data, like a TimeMachine or clone drive, like you should be doing, then really there is no need, just wipe and restore everything from the backup instead, that's what it's there for.
I never had a need or use for Disk Warrior to fix the file structure, however some who deal with a lot of users repairs (who don't backup) might find it required.
If you read enough of my User Tips, you will learn how Mac's work, how to keep things fast and how to fix things, make backups etc that don't require any third party software just about.
There is only one program I advise or have ever needed to speed up my Mac and that's because it removes all the caches (OnyX) and allows OS X to rebuild them, then it's only a repair program if the caches are corrupt and causing the slowdown to begin with.
Never in my 20 plus years have I ever needed anything else, because if adequate backups are made and the right kind, takes some preventative methods, the usually nothing goes wrong with a Mac.
OS X itself usually doesn't break, it's the quality of the medium retaining it's bytes on the hard drive that is the problem in a lot of cases. If the bits on the hard drive are damaged or not mapped off adequately enough then OS X and even programs will act funky or plain problematic, spinning beach ball effects and so forth.
Defragging software - Mac's typically don't need defragging because OS X writes most small files in one piece on the drive. Some have used defragging software successfully, but also many have problems defragging a "live" system, meaning one that's in active use. I do not advise one do this, because it doesn't take much for it to have a problem and your machine refused to boot. I've outlined a defragging method below that's much safer and gives a bootable copy so your not hosed and can't use the machine at all, like what can occur with defragging software.
What can cause a slowdown with Mac computers
Network related issues
You need to determine if it's your network solely that's to blame or is part of the problem. So disconnect from your network and attempt to like tasks offline to determine if your problem is computer or network related, it might be both even.
Take the machine to another network, use other computers on the same network etc., in your process of elimination. Consider your router is old, your Internet connection speed isn't adequate, you have leechers, bad cables, electronic interference or too many Wifi's in the area fighting over channels.
It's really difficult to diagnose network related issues because a lot of it is invisible to the eye and special "sniffing" software is required to see what's going on that one can't see. I suggest a good book on the subject, also I won't recommend certain software because of the sophistication and complexity, plus dual nature of most of them which can get a newbie into further trouble. If you need help I suggest hiring the local PC/Mac computer repair shop to come over and take a look at what's going on with your network.
The purpose of this User Tip is to focus on computer issues, if your problem only occurs while your online, then see this User Tip of possible WiFi/Internet related issues if or before you have eliminated the computer as the source of the slowdown. If it's not enough, call in a professional.
There are many broadband speed tests online that one can test their download speed is what the Internet Service Provider is providing.
Note the further away the test server, the slower the result obviously.
Also you might want to bypass your router by disconnecting it and hooking the Mac directly to the modem with a Ethernet cable, (power off/on the modem).
Secure your network from the leechers
Boot drive is getting filled up
Open Activity Monitor in your Applications > Utilities folder and look at your Disk Usage.
If your boot drive is near full, anything over 80%, then you need to consider using a Storage Drive to offload some of your User Files located in your Documents, Pictures, Movies etc folders. I would start with Movies as they tend to be quite large and moving a lot of them will free up space on your boot drive. You'll need to reboot for OS X to recognize the new drive space, it doesn't update automatically as you remove stuff.
See "Storage Drive" here
Not enough RAM for all your open programs and files
Open Activity Monitor in your Applications > Utilities folder and look at your System Memory.
The more green means more free memory that's not being used and is empty.
The more blue means that memory has something in it in case you need what's in it again, but it is available for new uses.
If all you see is Red and Yellow and the Page outs and Swap is high, then your using your slower storage drive for extra memory, buy more RAM or reduce your programs running and their open files so you have some green.
Activity Monitor has a cute trick to make it's icon in the Dock show your memory use, this way you can keep a eye on things that you don't go over.
Too many programs running at once, runaway process
Look in your Activity Monitor, click on CPU and "All Processes" and attempt to gauge if you have to many programs running that's taking all of your available RAM and thus "swapping" memory with the slower boot drive. Programs have memory requirements, too many at once will over load your RAM and slow down your machine, and in 10.7 slow down your boot time. Check the log in items in 10.6 too in your System Preferences > Accounts.
For a runaway process, sort by CPU % and see what's taking up a lot and attempt to force quit it or reboot the machine, check for updates, it might be a bug that got fixed.
This site can assist in tracking down the program responsible for the runaway process
If it's a third party program, it can be undated and/or reinstalled over itself and hopefully correct the issue.
Some Apple programs can be reinstalled alone, option click on Purchases in AppStore or only via a Reinstall Just OS X method
Always on anti-virus/anti-malware
Always on anti-virus has a continuous running process consuming your CPU cycles to check on everything your doing to make sure your not going to get malware. This might sound great and all, but the odds your going to get Mac based malware are not exactly nil, but not enough to warrant a always on type of solution.
The dangers are the always on anti-virus is more likely to cause problems than the slim chance you will get a Mac based malware. Apple tweaks OS X under the hood with a Software Update and then blamo, your machine is borked and the always on anti-virus becomes unstable and your machine acts slow etc.
Since 99% of the time the malware your going to get on a Mac is based on Windows and doesn't effect OS X, you can choose to uninstall the always on anti-virus for one that you run as you need it use it, like the free ClamXav. This way once in awhile you run a scan to clean the infected files before passing them onto your Windows using friends.
Some businesses demand all computers run a anti-virus solution, ClamXav will allow you to comply with this as you don't want to be passing those infected files at work and your machine will operate better with less slow down issues.
I advise you to read this on the current state of what to watch out for concerning Mac malware and usage behavior.
Something is wrong with your machine
Caches hold data OS X uses to load things faster, if these get corrupted/overbloated then slowdown issues may occur. By clearing all the system and other caches they will rebuild with only the latest needed data, thus clearing the corruption issue and return a speedy machine again.
Clearing the caches is only a troubleshooting step, it's not something that requires to be done often (except for privacy reasons) as the caches are designed to speed up the machine's operation.
Run the #12 OnyX routine first to hopefully clear the issue right away without requiring other Steps to be performed.
Disconnect all external hardware so your machine is as close to factory as possible, then go through these Steps to try to determine the cause.
If you exhaust that, a complete backup, zero erase and install of everything (possible on a new drive + more RAM) may be in order
Bad/failing sector effect on hard drives causing beachballing and slow read speeds.
Sectors are sections on your hard drive (SSD's no need) that store data in magnetic media, they can sometimes fail and thus cause problems as OS X, programs or files lose part of themselves and thus cause issues, cause corruption of your data, even files get lost.
There is a automatic mechanism in place with driver software to map off these failing sectors and place your data on another sector, but it takes time and causes slight beachballing effects, slow reads etc.
DO NOT move a computer with a hard drive while it's operating, the heads can strike the platters and ruin your data and cause bad sectors as well as the ones that occur by slight defects that all hard drives have.
Unfortunately to cure this problem requires a complete erase with a security option of "zero all bits" on the problem partition or drive and install of OS X, programs reinstalled from original sources (no TimeMachine or clone restores) and files returned from backup.
It's not a easy thing and if your not capable, seek the services of a local PC/Mac specialist. Usually in cases like this the hard drive is replaced as to solve all other possible issues as a failing drive can also cause bad sector and data corruption to appear.
However if you know what your doing then before you install OS X, use Disk Utility and select the Security Option Zero or move the slider one selection from the left and Erase before or during the drive or volume formatting option, then installing OS X fresh again. You might have saved yourself the cost of a new drive.
Could be malware, but likely not on Mac's
Mac malware is very rare, it's more likely your slowdowns are occurring because you installed some anti-virus or another and it got outdated with a OS X update.
I advise uninstalling all anti-malware programs and opt for a run as you need it program instead like the free ClamXav.
Most of the time it's going to catch Windows based malware anyway, which won't affect OS X.
However we have some malware scares in the past, review this and take some hardening steps.
Hardware isn't up to snuff with OS X or software
Make sure your hardware is beyond the minimal requirements of the operating system version hardware requirements Apple gives.
This tends to also to apply to a lot of software, minimal specifications outlined by the software maker are usually below par.
IMO, if your machine originally came with 10.5, the machine is getting a bit dated to begin with and unable to handle 10.7+ adequately, 10.6 (Software Updated to 10.6.8) is likely the best performance choice for most original 10.5 issued era machines.
You may decide to forgo upgrading to 10.7-10.8 and simply get a new 10.8 machine rather than suffer under 10.7-10.8 with a slow machine and have to buy all newer versions of some software to replace your PPC based ones that 10.6 was running using Rosetta.
It's always good to get a lot of unbiased opinions how well a operating system upgrade runs on older machines before taking a upgrade leap.
Increase your RAM or replacing your boot drive with a faster one and fresh installing OS X + programs and returning just user files from Storage Drive usually can cure a lot of performance issues all at one.
10.6 is faster than 10.5 or 10.7 (not a typo!) consider going back to 10.6 on your machine (only if it had 10.6 on it previously).
In my opinion, 10.7 and 10.8 are better off on boot SSD's than boot hard drives, because of the Recovery HD and those OS's tendency to require more drive I/O performance, which SSD's provide.
Restored from TimeMachine/ full clone restore
What a great idea TimeMachine is, restoring a boot drive or migrating a user account from one device to another. Usually for a lot of folks this works out well, however a lot of time TimeMachine restores falls right on it's face and you get a slow or problematic machine soon after the restore process.
In the "Fresh install OS X" method, everything (except the users unique personal files) are installed from original sources so they are pristine, clean and operating correctly, it usually coincides with a Zero Erase method for hard drives to eliminate bad sectors in advance.
If you've read about the problems with bad sectors on hard drives losing parts of OS X, programs or even users files, then you can see if you had a corrupted sector on your original boot drive that was causing your data corruption issue and then TimeMachine backed that corruption you restored from that to a new drive/fresh install of OS X, then you can see why your problem appears to return again.
In 10.7+ Apple has made the Users/Library folder hidden, this complicated matters as any corruption in there gets transferred during a user account only migration.
How to escape the trap
Ideally it's best to have more than one form of backup, I advocate one use a bootable clone in addition to using TimeMachine or in place of it. With clones one has the ability to access the clone drive directly like any other.
So if you want to dig inside and easily only get your files out you can, files are easy to comb through to eliminate corruption, programs and OS X is not.
Also clones are bootable so in case you boot drive dies you can run off the clone drive.
To have more than one backup method and even have one's files on DVD's or a separate storage drive.
Important thing is to have your personal files in a state that they are easily accessible so you can restore just those and pick and choose what you want, not just a all or nothing approach which Setup/Migration Assistant or full clone restores do.
Since one needs diversification of their backups anyway, I suggest you read this
Want to Defragment and Optimize your Boot Hard Drive? (SSD's no need)
Performance Tip (advanced)
Ideally with boot hard drives specifically (SSD's no need), if one can keep them below 50% filled, and all data on the first 50% of the hard drive, they will perform their best because the sectors get less on each track on the second 50% of the hard drive. I know this sounds unrealistic for some to keep under 50% filled, but when considering purchasing a computer, to get twice as much boot hard drive space as they are ever going to use.
The second 50% can be partition and used for extra space, however whenever OS X has to access the second half, just realize the performance is going to suffer more than on the first 50%, so the less it's used the better if your interested in having a speedy machine.
SSD's work differently, all locations on the SSD can be accessed with the same speed. However they are very expensive in larger sizes, don't have secure date deletion ability and older Mac's don't have the SATA connections to take advantage of the speed. Most likely a waste of money on a anything earlier than a 2010 Mac.
To achieve having OS X and everything back onto the first 50% of the hard drive, may require backing up and erasing (with zero erase) and reinstalling OS X, then any and all programs possible, then finally files returned in priority stages, rebooting in between and gauging the drive space taken as not to exceed 50% filled.
Once can also clone to a external drive using Carbon Copy Cloner, option key boot from the clone, reduce files, then erase the internal boot partition (with zero's) and then reverse clone from the external. This method is far superior than a defrag as it also optimizes OS X and programs near the fastest part of the drive, and Users files, which expand and contract often into the lower parts of the drive were performance can suffer more or less depending how much data a user has on the drive.
Note: This procedure only works for hard drives with no problematic sectors, and no software issues, review those possible causes FIRST, before defragging.
iPhoto/Aperture database corruption/too large
If any of your photos's are corrupt, this could cause slowdown issues in these programs, no other way but to do a tedious process of elimination to find out what roll is causing the issue. Reports I see if you import photo's from a new camera or source might be the cause.
iPhoto/Aperture loads all your pictures at once (unless separate Library/Vaults are used), so if you have a lot of pictures and not enough RAM or a older machine, or one that isn't very powerful, your machine is going to act slow.
Also your boot drive may be filling up on the second 50% of the drive where it's slower. Also your boot drive may be too slow in it's read/write speeds to handle your huge photo library, you may want to reduce the photo's and place the unused ones on a external storage drive (and make a backup of that too) so to reduce your load on your boot drive and machine.
Consider a external hardware based RAID 0 (wtih another backup as RAID 0 can fail more often) or SSD storage drive on the fastest connection your Mac has (Thunderbolt or Firewire 800 preferred), keep the bulk of your files there and off the boot hard drive so it remains fat and speedy.
If you hold the option/alt key while launching iPhoto, you can access the iPhoto Library on the faster external RAID 0 drive.