For what it's worth, I just re-tried switching the display mode from borderless window to fullscreen, and lo and behold, it works! The framerate is back to pre-HF21 levels, and the game is eminently playable again.
I tried the same trick when the frame rate problem first arose, but to no avail. I don't know if it's the subsequent hotfixes or the fact that this time I made the switch in the starting menu, before I'd loaded the save file, but anyway I'm back to the races!
2020 M1 MBP, 16GB RAM, mostly maxxed settings, using a Dell U2421HE monitor
Interesting. Ok let me download it ... again.
