Why has my iMac recently become very slow?

I don't see any obvious issues however your machine is quite old and may have a hardware issue. I would recommend locating Disc #2 of the original DVDs that shipped with the computer and use Using Apple Hardware Test - Apple Support. Run AHD in EXTENDED MODE 2-3 times back to back. The reason for running it multiple times is AHT does not always find errors after the first or second pass. However when errors are found they are correct. If you cannot find the discs then contact AppleCare and if they are available you can get them replaced for a nominal cost.

If you don't want to do this then take the machine to an Apple Authorized Service Provider (an Apple Store will not work on it due to its age) and they can do a professional diagnostic. You can begin finding an AASP in your area by clicking https://locate.apple.com/country

The Photos application is very active. Have you enabled the iCloud Photo Library? That could be slowing down things as it uploads all of your library to the cloud. How long has it been since you first started Photos?

A couple of items that are know to slow down Macs are Google's Chrome and Drive. Only launch Google Drive when you need to upload to it. As for Chrome I'd get rid of it and use either Safari or Firefox. Both are more efficient that Chrome.

When you see a beachball cursor or the slowness is especially bad, note the exact time: hour, minute, second.

These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.

Launch the Console application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

The title of the Console window should be All Messages. If it isn't, select

SYSTEM LOG QUERIES ▹ All Messages

from the log list on the left. If you don't see that list, select

View ▹ Show Log List

from the menu bar at the top of the screen.

Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.

Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.

Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.

The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.

Please don't indiscriminately dump thousands of lines from the log into this discussion.

Please don't post screenshots of log messages—post the text.

Some private information, such as your name, may appear in the log. Anonymize before posting.

When you post the log extract, you might see an error message on the web page: "You have included content in your post that is not permitted," or "The message contains invalid characters." That's a bug in the forum software. Please post the text on Pastebin, then post a link here to the page you created.

If you have any other suggestions I'd be delighted to hear them.

Suggestions regarding what exactly? If it's about backing up your photos you have Time Machine enabled and that does a good job. You might get an external HD with FW 800 capability and clone your boot drive to it and then update the clone maybe once a week or month. If you experience an crash of your boot drive you can boot from the EDH clone and continue running until you get the crash fixed.

Purchase an external connected and Mac formatted Flash drive OR externally connected USB, Thunderbolt or FireWire 800, Mac formatted hard drive. Then, purchase, install and use a data cloning app, like CarbonCopyCloner or SuperDuper, to make an exact and bootable copy (clone) of your Mac's entire computer system and data.

Typically, iTunes and iPhoto libraries are the biggest users of HD space.

move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.

If you have any other large folders of personal data or projects, these should be archived or moved, also, to the optical discs, flash drives or external hard drive and then either archived to disc and/or deleted off your internal hard drive.

Moving iTunes library

http://support.apple.com/kb/HT1449

Moving iPhoto/Photos library

http://support.apple.com/kb/PH2506

Moving iMovie projects folder

http://support.apple.com/kb/ph2289

Things to consider before moving your iPhoto/Photos Library Folder to a new or external location like an external hard drive.

If you make movies on any iDevices using iMovie for iOS,, then transfer the video footage, the IOS version of iMovie saves the footage as a movie file in IPhoto for IOS and will automatically get transferred to iPhoto for the Mac when you upload the video from your iDevice.

Newer versions of iMovie will work and link those video files found in your iPhoto Library on your Mac, but those links can be lost if you move your iPhoto library and you will not be able to relink that video afterwards as the current versions of iMovie seem to not have a relink option for the video portion of the files (ironically, current versions of iMovie HAVE the ability to re-link the audio files from the video footage, though (The inability to re-link the video files could be a possible bug or oversight in current versions iMovie).

The lost video links show up as "blacked-out" video blocks with no content.

Before moving the iPhoto/Photos Library

If you make movies with iMovie using iPad or iPhone video then 'Consolidate' the files before you finish. This will gather (albeit by duplicating) all the relevant files in the project in one place. After consolidating/duplicating all of the audio and video footage to a seperate, independent location,it should be safe to move your iPhoto library.

The potential way to circumvent this issues maybe to try and import iPad and iPhoto video directly into iMovie which would be another solution.

Good Luck!

Old Toad is a genius! I turned off the iCloud library for my photos and now my iMac is back to rapid speed!