How do I speed up my iMac with OS X Yosemite?

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.

ajstenger wrote:

How do I speed up my iMac with OS X Yosemite 10.10.5?

Do not install anything claiming to do that, for one.

Back up your Mac if you have not done so already. To learn how to do that please read https://www.apple.com/support/backup/. To learn how to use Time Machine read Use Time Machine to back up or restore your Mac. The reason for creating a backup is that the symptoms you describe could be indicative of an impending hard disk drive failure, so securing your data is a primary concern.

Then, shut down your Mac and restart it in "Safe Mode". To learn how to do that please read Use safe mode to isolate issues with your Mac - Apple Support. Compare how your Mac works in Safe Mode to how it has been.

Then, restart normally. If the same problem returns, please read the section If an issue doesn't happen in safe mode for Apple's recommended actions.

Finally, you can temporarily create a new User Account, and log in to that account to determine if the problem also exists when logged in to that Account. To learn how to do that read How to test an issue in another user account on your Mac - Apple Support. When you are finished troubleshooting, you can remove the temporary User Account by following these instructions: OS X Yosemite: Delete a user. Before removing it, confirm you don't need any of the files you might have created in that Account.

Let me know what you determine.

You could post your EtreCheck output and let us see what your system configuration looks like.

It is very possible Kappy's links will help you sufficiently.

If your system is clean with respect to 3rd party additions, and undesired adware, then the next avenue is hardware upgrades. The 2 most effective is maxing out your RAM (<http://Crucial.com> and <http://MacSales.com>) and replacing (or supplementing) the rotational hard disk with an SSD (<http://MacSales.com>)

If your memory is near full during normal use a lot of the time, or if you have less than 4 GB, more memory will often help. You can see how much memory your computer uses in normal operation by opening Activity Monitor (the easiest way to get there is to search Spotlight), and click the Memory tab.

Note that memory is different from storage. Memory is a short-term cache for files that apps are using currently, while storage is your hard disk — it holds all your files long-term. Having less than 20 GB free on your storage can also negatively impact performance. Check in  menu > About this Mac > More Info and click the Storage tab.