Mac OS Big Sur - mouse stutter (not a Bluetooth problem)
I was having the same issue. Problem was being caused by using a secondary monitor that was connected to the USB3.1 slot of my iMac. (Display-to-USB3.1) For me the problem was solved after reconnecting/changing it to a different USB slot. I also can confirm it has nothing to do with the mouse since i was experiencing the same issue with a WacomPenTablet.
So it definitely has something to do with USB ports and Displays.
I Hope this helps.