run into problems with overheating and battery draining fast since big sur update as well. checked acitivity monitor, turns out a process named secd is mainly the reason, constantly using my CPU >50%. found out it is somehow linked to the keychain. deleting the local keychain and restarting solved the problem for me - no battery drain any more. (read this: waal70blog.wordpress.com/2019/12/16/accountsd-and-secd-high-cpu-usage-on-catalina/). I would only have to type in all passwords for internet accounts and set touch ID again. make sure you use icloud keychain to not lose any important passwords. anyway, before you do this, check your activity monitor. every new installation forces spotlight to index, wich could also account to high CPU usage. But this should not last longer than a few days.