Slowness of MAC desktop

Try and boot the Mac in the safe mode to see if it is caused by an external piece of software

Start up your Intel-based Mac in safe mode

1. Turn on or restart your Mac, then immediately press and hold the Shift key until you see the login window.
2. Log in to your Mac.
3. You might be asked to log in again.
4. On either the first or second login window, you should see “Safe Boot” in the menu bar.

if it does not persist in safe mode, the issue is caused by an external piece of software, You might want to evaluate the software installed to your Mac

And if the great advice so far doesn't help...

Are you running any VPN, Anti-Virus, or Cleaning apps?

We need to see what all is running, a report from this will not display any personal info...

https://discussions.apple.com/docs/DOC-6172

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Thanks for Old Toad’s etrecheck instructions…

https://discussions.apple.com/thread/255121100?answerId=259529290022#259529290022

Use the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...