Why is my iMac so dang slow!!!?

A Troubleshooting Protocol to Identify Problems or Fix macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Disconnect all third-party peripherals.
3. Resetting your Mac’s PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences.
6. Start the computer in Safe Mode. Test in safe mode to see if the problem persists, then restart normally.
7. Use Apple Hardware Test to see if there is any hardware malfunction.
8. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
9. Repair permissions on the Home folder: Resolve issues caused by changing the permissions of items in your home folder.
10. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
11. Download and install the OS X El Capitan 10.11.6 Combo Update or 10.12.6 Combo Update or Download macOS High Sierra 10.13.3 Combo Update as needed.
12. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
13. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continue button.

If none of the above helps then make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple.

Ways to help make a slow Mac faster

1. 17 Reasons Why Your Mac Runs Slower Than it Should
2. Slow Mac Performance? This Article Solves It!
3. Fix slow start-ups in OS X | MacFixIt - CNET Reviews
4. How to fix slow shutdown and startup times. | MacTip.net
5. 6 Easy Tips to Speed Up OS X Yosemite on Your Mac
6. OS X El Capitan- If your Mac runs slowly
7. Visit The XLab FAQs and read the FAQ on Performance
8. Tips to Fix Issues that affect Mac Performance
9. Avoid using any third-party cleanup software. Usually, this software does more bad than good. You don't need it. All computers become slower over time even under normal use. Experienced users erase the hard drive and do a clean install from scratch from time to time; or whenever installing a major OS upgrade. Doing so means you must maintain regular and multiple backups.
10. If you have enabled iCloud Disk and are storing your Documents and Data in iCloud, then consider turning that off and signing out of iCloud. It can slow down the computer considerably. Please see the following from the user, fotomac: "The solution was to SIGN OUT of iCloud and my problem STOPPED! NO MORE SPINNING BEACHBALL! My computer's speed increased to what it should be and all my Apps now work!"
11. Add more RAM or cut back on the number of concurrently running applications and utilities. Remove unnecessary anti-malware software and any software that promises to clean your Mac. Check for runaway processes: Runaway applications can shorten battery runtime, affect performance, and increase heat and fan activity. Also, visit The XLab FAQs and read the FAQ on dealing with The Spinning Beach Ball of Death.
12. The Ultimate Fix: Backup everything, erase the drive, reinstall OS X, and restore your data from the backup. Reinstall third-party software from original media/scratch.

download and run Etrecheck. Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Here's the BIG issues with all, but the topped out, 2013-2015, slimline, 21 inch screen iMac.

The 2013-2015 slim profile 21 inch screen iMacs that Apple had offered are, IMSO, "downgrades" from the previous 2009 thru 2011 21 inch iMacs models.

The 2013-2015 slim profile, "slimline", 21 inch screen model iMac are, basically, sealed up, glued up, basic computing appliances, now, with only the "top of the line" with all the top options (top i7 CPU, 16 GBs of RAM, 256 or 512 SSD) being the only viable option to use as a standard to medium professional grade, work related, iMac with just enough power for professional and creative use.

There is nothing in the 2013-2015, 21 inch screen iMacs the is user upgradeable OR serviceable, any longer.

Slower laptop 5400 RPM hard drive/fusion drives, limited RAM (the 2010, 2011 i5 CPU models and higher could take up to 32 GBs of RAM).

These 2012-2015 slimline 21 inch screen iMacs are limited to just 16 GBs of RAM.

Slower 5400 RPM laptop standard/style hard drives are 33% slower than the equivalent storage size in a "desktop standard" 7200 RPM hard drive.

This means 33% slower read/writes to the drive, in addition to other system delays.

Also, like laptop models, these older 2013-2015, 21 inch screen iMacs no longer use dedicated, discreet and independent GPUs.

These older slimline 21 inch screen iMacs use the "so-called" integrated (soldered) Intel Iris ”Pro" GPUs, now!

Meaning that in any of the newer 21 inch screen model iMacs, if you get one with the max 16 GBs of RAM, a little bit to quite a bit of that installed RAM is being used for the GPU's VRAM!

Meaning that the GPUs in these newer 21 inch screen model iMacs is stealing away physical RAM, for use by the GPU, for the GPU's VRAM, that could be needed for any other possible RAM intensive tasks depending on the apps being used or what combination of apps is being used together.

The GPU will automatically steal whatever the amount of physical RAM the the GPU needs for any type GPU related task.

This would not be as big an issue if you could install more than 16 GBs (which there is no way to do this in these 2013-2015 , 21 inch screen iMacs as Apple has soldered every single component to the logic board).

The 2012-2015 baseline 21 inch screen iMac models only come with 8 GBs of RAM standard.

New versions of the Mac OS need a base MINIMUM RAM of 8 GBs, now.

So, 8 GBs of RAM is really not enough RAM to run both the OS AND any additional applications.

Especially the type of applications that are SERIOUS hardware resource intensive!

To get a better performing iMac, now, you would have to spend the money for all of the top end options for the new 21 inch screen model iMacs to ensure better longevity and "futureproofing"

OR

You would really have to graduate up to the more expensive 27 inch screen iMacs that still have 7200 RPM hard drives/fusion drives, independent GPUs and user installable/upgradeable RAM.

I am NOT a big fan of these 2013-2015, 21 inch screen iMac models, at all!

Other than a top shelf version 21 inch screen model, the other 21 inch screen model options are like a Mac Mini with a screen, now, or like a laptop/desktop hybrid, sealed up appliance.

IF you have had this iMac for longer than 14 days, then sell it and get as much as you can for this iMac and go for a totally option maxed out 21 inch screen iMac with either the 256 or 512 GB SSD.

This is the only way to make these newer slimline, 21 inch screen iMacs faster.

To greatly improve the speed and performance of your 2013, 21 inch screen iMac, you are going to have to spend some significant more cash for this to happen.

My solution is to go the less difficult plug and play (somewhat) route.

Purchase an externally enclosed USB 3.0 or Thunderbolt connected SSD.

Format the SSD for Mac OS extended (journaled) with GUID partition scheme and then purchase data cloning software, like Bombich Software's CarbonCopyCloner or Shirt Pocket's SuperDuper to make a bootable clone of your current system to the SSD and boot and run your iMac, exclusively from the externally connected and enclosed SSD.

Your iMac will run A WHOLE LOT FASTER from an externally connected and enclosed SSD!

You can either opt to use your internal iMac's hard drive to just store data OR purchase, yet, another external USB 3.0 or Thunderbolt connected 7200 RPM mechanically spinning hard drive to use as your primary data storage and data backup.

If you are from the U.S., here is a good Mac/Apple source for external SSDs and standard 7200 RPM hard drives here.

https://eshop.macsales.com/shop/external-storage

You can call them to speak with a sales/technical rep or contact by email

RAM and GPU are still problematic in these 2013-2015, 21 inch screen iMacs, though.

There is nothing to be done with that!

Good Luck!

Apple sold the 2015 model iMacs into early 2017, when the new and, somewhat improved 2017, 21/27 inch screen models were introduced, but some of the flaws of all tiers of the 2017, 21 inch screen iMacs still remain.

You still need to purchase a 21 inch screen iMac with ONLY a full SSD as Apple is still only using 5400 RPM hard drives and pairing smal storage flash drives to 5400 hard drives in the 21 inch screen model Fusion drives.

The use of 5400 RPM hard drives/Fusion drives is a bad combination.

For the 21 inch model iMacs, I can ONLY recommend a mid to high tier iMac with a minimum of 16 GBs of RAM and with any of the full SSD options of your choice. A 256, 512 GBs or 1 or 2 TB full SSD.

Without the SSD and more RAM, you will end up with a slower iMac for what you are paying.

What iMac screen size is this iMac? 21.5 inch screen or a 27 inch screen iMac.