Just giving an update, might help some people I upgraded from Catalina to Big Sur and had the same slowness that everyone here is complaining about. I have a late 2015 iMac with a fusion drive. At the suggestion of Apple support I created a new user with admin rights and started transferring all data from the existing user to the new user. This was a real pain but I think it's easier than doing a complete new install It's now a month in and I see no issues, my mac is as fast as it always was with the new user. When I try to login with the old user I still have the same slowness. Just for reference I'm a software developer and sometimes have multiple cpu/memory intensive applications open at the same time. Right now I have Eclipse, Intelij, a tomcat instance and a SQL server on docker (don't ask...) running with no issues!

@jonathon-b  this means I have to wipe the mac clean and start again? How is this any better than windows? The thing that Mac has over windows is that it never crashes and doesn't slow down from year to year. Since I "upgraded" It slowed down to a crawl and it crashed twice.

@UBogun That's what's frustrating about this. It's not CPU processing or memory consumption that's causing this slow down. If it would be I would be able to pin it down to a certain application or activity that's causing it. Like you, I still have most of the RAM free and CPU is not really working hard either. I get the feeling it has something to do with accessing the hard drive but I might be wrong

@Lostinlodos  I just created a new user and gave that user admin rights. For the most part it's easy enough to migrate but if you have licensed software you now need to enter the license again For me the hard part is that I have a clients microsoft remote desktop setup and that can't be migrated. Which means I'll have to ask the clients admin to login in to my desktop to set it up again, which is a real pain. Everything else is just copying the files over and since the new user has admin rights you just need to run sudo with the cp commands

I have the same issue as mentioned by everyone here. I contacted Apple support , they suggested creating a new user with admin entitlements to see if it's a hardware issue or a profile/data issue. Surprisingly, that worked! It's still going to be a pain in the neck to migrate 4 years worth of stuff to the new user but at least I can continue working