@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.

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!