How do I fix my slow running iMac?

You have one app, Adobe Elements 2020 Organizer.app, that's taking up 97% of your CPU cycles. Itr's part of your Photoshop Elements 2020. Make sure you have the latest updates for that app.

You might try the following: boot into Safe Mode according to How to use safe mode on your Mac and reboot normally. Then test to see if you're getting that app using so much CPU with Disk Utility.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Reply

If the problem returns please do what Ronasara suggested. Here are some instructions:

Please download and run Etrecheck.  The free version is sufficient. Be sure to give it Full Disk Access.

Copy the report as shown in this animated screenshot

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.

Thank you for including the EtreCheck report. That provides useful information. Here is what I see. You have an older, slow mechanical HD and a Fusion Drive. The Fusion Drive should compensate for the slow mechanical drive. It is a small, fast drive. Unfortunately, sometimes these two drives become disconnected and you are left using only the slow mechanical drive. I would install an external SSD drive , which is much faster to function. Format it like your HD is using Disk Utility (only). Then download and install the operating system you are using onto the new drive. You then will be prompted to set up the new drive and during the process it will ask you if you want to open Migration Assistant and import data. Say yes and let it do so. The new drive will be set as the startup drive and its speed will be lightening fast compared to your old HD. I know because this is what I did.

One other point. Startup will be slow if you have the computer set to open a lot of apps on startup. Check the System Settings app and click on the General link on the left, then Login Items. The new window will show you how many apps are set to automatically open at startup. If you can turn most of these off, it will speed up your startup.