HOPE Just wanted to add to this thread and say there is hope, because I thought there was going to be none after I installed Big Sur and waited for days. I installed Big Sur (v 11.5) a few days ago... install took a few hours and then my system slowed to an absolute dead crawl. Pushing one key on the keyboard would result in either nothing or multiple key entries... a patient and slow key press finally got me logged on to my user account (there's only one)... once there, my mouse would take ~2-3 seconds to respond, was jagged, laggy, overshot the destination - in other words, impossible to control, as if every system resource was maxed out... mouse clicks were super problematic, non-respondent and delayed... the system was almost unusable. I read comments in this thread, saw that lots of others were experiencing similar problems... I told myself to wait a day or so to see if some background processes were still resolving... all the system resource apps (Activity Monitor, Sys Info, etc) showed almost no resource usage, which seemed strange, so I waited. For two days... nothing changed... sys was still bogged down... Woke up this morning. Checked Reddit, saw someone mentioned to reset the Systems Management Controller (SMC) by powering down and unplugging for 15 seconds and then powering back up. Check here: https://support.apple.com/en-us/HT201295 I powered down, unplugged and then for added good measure, unplugged all of my external drives (someone had mentioned USB controller issues, too)... it worked like a charm and true to some other posters I've seen elsewhere - the system feels faster than it ever was... HUGE sigh of relief... My sys specs: iMac (Retina 5K, 27-inch, Late 2015) Processor 4 GHz Quad-Core Intel Core i7 Memory 32 GB 1867 MHz DDR3 Graphics AMD Radeon R9 M395X 4 GB Good luck! Hope it works for you, too.