To fix your Big Sur Mac, boot it up in Safe Mode by following these steps:
Shut down your Mac and wait for 10 seconds
Start up your Mac while holding the Shift key
Release the Shift key after the login window appears on your screen
If there are no performance problems, reboot the Mac
You can also reset NVRAM to solve common macOS Big Sur issues.
It goes like this:
Shut down your Mac
Start up your Mac while holding Command (⌘) + Option + P + R
Release the keys at the second startup chime