Afterthought after rolling around my bed for a few hours: After installation, I'd first try to run the Combo, and if that's a go, I"d then run the security updates in chrono order, one at a time, to see where that leaves me.
I did that, doing the Update using the ⌘R approach. I started with the Security Update 005, restarted, and found there was a Security update 006 - so I installed it.
After another restart I did the Supplemental Update which upgraded Safari to 14.0.1
I found that everything was very slow between every install. Then something weird occurred. After a few restarts, things cleared up and I was back to reasonable speeds. It was almost as though Oral Roberts dropped by and placed his hand on my computer.
We now have the answer. After my re-install of Mojave, everything worked fine. However, after I installed all the updates (which included one more security update than in the list of three updates above), things slowed to a crawl....and I mean excruciatingly slow. I booted in Safe Mode and things are back to being quick.
What does that say?
What that says to me is you have a third party kernel extension installed on your system that does not
play nice with either the supplemental update or the security update or possibly the combination of the two.
Before you go through another reinstall, if you look in /Library/Extensions and, possibly, /System/Library/Extensions you should recognize anything that you installed on your own, and either delete the kext if it was placed by an app you no longer use or quit the apps that placed them. one at a a time, to see if you can pin your error down.
Athough things are running well, I am removing Extensions I don’t recognize (from /System/Library/Extensions) and, unless I’m just starting to lose my mind, I believe I am getting small increases in speed. There are some Extensions with absolutely arcane names so, until I know what they are, I’ll just leave them alone.
Curious observation: Throughout all of this, some apps could be counted on always to load quickly - my 32 bit apps.