I had this issue and uninstalling Avast solved it for me.

Thank you erikxy! Removing Avast fixed it for me. Computer is now running fine with Big Sur. Hope this works for others as well!

Same thing happened to me after updating to Big Sur. 2018 Macbook Air is now virtually unusable. Boot time is absurd, opening an app like Activity Monitor takes several minutes, and the computer freezes often. I tried setting up a new user account as someone suggested and barely managed to do so after about half an hour but it is just as slow. My very expensive computer is currently trash. Apple, please fix this!!!