iMac (late 2013, 8GB RAM) very slow, like molasses after upgrade to OS Catalina

Edward M. Baum wrote:

My daughter's iMac has become almost unusable after upgrade to Catalina. I understand from a reply to a previous post that such machines with a spinning HD were no longer optimized, but this situation makes her work almost impossible. It's hard to imagine Apple would not have issued a warning for such serious degradation of performance. Every move with the mouse is accompanied by the spinning beachball and a wait of many seconds.

Is there anything that can be done? Can there be other causes for the radical slowing down I should check on?

If not, what is the process for removing Catalina and installing the most recent OS that works well with the HD that came with the iMac?

I've run Disk Utility and it reported nothing amiss.

Many thanks for advice !!!

Ed in Brooklyn

To trouble shoot further you can:

— Try a SafeBoot https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc. Login and test. Reboot and test as caches get rebuilt.

In Safe mode third party system modifications and system accelerations are disabled, it removes malware, etc hampering smooth operation, however a reboot will put it back to normal mode.

This test will tell you if third party interference; extensions etc are not loaded in safe boot mode.

— Test issue in another user (or guest user) account  https://support.apple.com/en-us/HT204443

This will tell you if it a universal issue or isolated to your user/admin account. 

if no progress then you can easily revert from your backups.

Revert your Mac to a previous macOS version - Apple Support