A weird Windows 10 problem this morning. When I booted up the PC, it appeared very "sluggish". There was no obvious reason for this: CPU and disk I/O were all very low. On closer examination, the problem was with window refreshes: things were working OK in the background, but the contents of windows weren't being repainted for several seconds after. Switching away from a window and switching straight back seemed to "trigger" an update that should have happened all by itself.
Anyway, after a bit of Google searching, I happened upon a description of a similar problem and its solution that I don't think I would ever have come up with by myself. Somehow, the screen refresh rate had been changed from 60Hz to 59Hz:-
Sure enough, changing this back to 60Hz resolved the problem immediately.
I hope this saves someone else some pain (or saves me reliving the pain if it happens again and I have forgotten what the fix was !)