I’m sharing an iMac Late 2014 (the oldest computer that is said to work with Big Sur)
It was very slow so we fully wiped the computer and installed a fresh copy of Big Sur, and it was still very slow, laggy etc.
We left the computer on doing nothing for 2 days and now it’s fully fast as it was before – there are 2 days of background update processes that the mac doesn’t tell you about…
If you have the issue:
late 2014 iMac by apple running very slow , not working, broken, frozen, update failed
Try leaving it on for 2 days to finish installing.