Magic mouse not working after Big Sur upgrade

I had the same issue as others had with the bluetooth mouse after the upgrade. It would track for a few seconds then suddenly lag and I have to wait for it to catch up. Totally random lag, too. I tried all the fixes suggested including resetting the bluetooth connection, deleting the library files for the mouse and then restarting, yet nothing worked. I then saw a reply to the earlier thread about a display issue using up resources. I didn't have that issue, but I did notice an old MovesCount program that still started up automatically that was to support a Suunto running watch I used to use. The MovesCount program was always buggy and I don't use it anymore. Deleting the program fixed the issue.

Recommendation: if the other fixes don't work, look for a resource hog or a buggy app.