Had the same problem. 2020.5, Mojave 10.14.6 with latest Security Update. Everything normal yesterday, and today all sessions freezed every 5-seconds, with that damn beach ball(no problem with playback, just graphics).
Was relieved(kind of) to find this thread and see that it wasn't only my setup. It's odd that everybody seems to just started to have it today with no apparent reason.
After reading this whole thread I've followed the steps that seems to be working for some folks: trashed prefs, uninstalled and reinstalled ProTools, disabled my network(turned off wi-fi and made ethernet connection inactive). I've also opted out to send info to Avid at reinstall and used Little Snitch to block Pro Tools connections(it seemed to be phoning home). Finally, uninstalled UVI Drumreplacer demo, since it was the only plugin that(coincidently or not), I've installed yesterday.
Lauched Pro Tools, finger crossed, and thank God it's working fine again!
After that I've turned my ethernet connection back(still kept wi-fi off and little snitch restrictions, just in case) and it still running fine.
Not sure what did the trick for me, but I'm glad it did. Thanks for all the tips.
Hope this help someone, and Avid, please, do your homework cause this is serious!