All 3 of my desktop computers have 16GB of physical RAM and resource monitor under task manager only shows ~22% of it being used so that's not likely the issue.
I rebooted the offending machine then loaded up all the programs that were running on it before and while the lag not nearly as bad, it is still there. I'm just going to chalk this up to the mysteries of Windoze.
Huh, I just noticed that my Windows Experience score dropped to 2.6 and the reason given was the performance of the Aero Desktop... I don't recall what it was before - who the heck pays attention to that score? - but I do remember it was limited by my HDD, not the GPU (besides, twin GTX 1060s are way more GPU than is needed to run Win10). Anyway, a bit of a mystery, but not a crisis so I won't worry about it.