The issue happened after I updated Firefox, Google Chrome, and Opera. I did it via clicking on "About (insert browser name)", and then doing the update via the download process. Maybe I should have first downloaded each new version of the software, and then do the installation myself.
In actuality, the issue might have reared its ugly head last Saturday, when I ran Tech Tool Pro. Two of its tasks are Volume Structures (Tests a volume for directory corruption) and Volume Rebuild (Rebuild and repair a volume's directory). When I applied those two processes to the Data and System Volumes on my Mac Mini, it ran definitively slower, taking an awful long time (WAY longer than I have ever seen). Both Tech Tool Pro and Disk Utility did not indicate anything was wrong with those volumes, so it would have been difficult to pin down.
On Tuesday, I did run ClamXAv to check for any viruses/Malware (ran Malwarebytes also), but nothing nefarious was found (that's always the case for both of my Macs).
In any event, after I had the issue on Thursday (machine started up real, real slow after applying those browser updates), I thought it would be best to do a clean, fresh installation of the latest version of Catalina, followed by a migration/copying of needed files, folders, apps, settings, etc. from the SuperDuper! backup I did last Saturday. (I make full, complete backups ever time, not snapshots). Today everything went very smooth with Onyx, Tech Tool Pro, and SuperDuper!. In fact, the machine feels snappier. (To be safe, I have not applied any of the browser updates, including the one for Brave (my primary one)).
I actually had no issues with OS 10.15.6, and until the Supplemental Update for OS 10.15.7 arrived last week, no issues with OS 10.15.7., I wonder if the Volume-related issues Tech Tool Pro was "dealing with" last Saturday had to do with that Supplemental Update.
I think I'll run Tech Tool Pro on the two SuperDuper! backups I made earlier today, just to insure things are OK. Seems good so far, though.