For me, it is not bad when running, but the boot is appalling. I have a 27" 2019 iMac Pro, with 64GB, and a 4TB SSD. Previously it took about 22 seconds to boot (slightly longer than my 27" I7, fusion), but now it takes over five minutes to boot. It is the section from about 40% to 50% in the progress bar, it just stops and walks forward at a tiny fraction of a mm/minute. The I7 conversely only takes perhaps a second longer. It's as if something in the boot doesn't actually like something about the Pro. Once it has booted it almost works OK. Just a couple of applications with issues.

After a long session this morning with Apple support, they have suggested I switch back to Catalina. The rep said that they were having a lot of problems with Big Sur, and it'd probably be several months before it was working properly. What I've decided to do is add a small external SSD, and install Catalina onto this, and use this to boot. Then can test at intervals as updates come along for Big Sur, to see if it starts to work acceptably.

To the user saying the machine where they did a clean install is OK, after my initial problems, I did this. Reformatted , and did a clean install. The problems remained. It seems to be some particular hardware combinations that are having problems. Possibly Apple did not get the proper beta testing done with Covid. On a 2017 i7 Fusion machine, Big Sur for me runs fine, On my 2019 iMac Pro using an SSD, it does not. On the former it does not seem much different from Catalina. Boot is possibly one second slower, but programs run OK and really everything is fine. On the latter, I used to get faster booting 30+ years ago, loading using a tape!....

OK. I've found three things that seem to really cause big problems with 'Big Sur'. The first is if you have a Wacom driver installed. This seems to completely screw the boot, Many minutes delay at boot. Answer here is to remove the driver and install 'afresh' (don't 'upgrade'). The second is certain anti-virus programs. Again remove, and install afresh. It seems that the driver they use to connect to the filesystem during boot, doesn't upgrade correctly, and has issues under Big Sur (a few like Avast, don't seem to work correctly even with the latest version). The third is large numbers of external drives. Something seems to go wrong with the USB handling for these, with lots of USB stall messages during boot. Now what is strange here, is I added an external USB3 powered hub, and now boot with this switched off. Once the system reaches the login, turn it on. Lo and behold, boot in under 20 seconds, versus over 5 minutes!. The drives all work perfectly. It appears that something in the USB/filesystem checking during the actual 'boot' process goes wrong when there are more than a certain number of drives attached. Attach them after boot, and it works great. Reinstalling the Wacom driver, and switching the drives on after boot, takes the boot time down by about 15 times. The system also then seems to run properly. Things open quickly, and don't glitch. Except for a few packages that don't run completely right, the system is now usable again.

Upgraded a couple of days ago to 11.2.2, and now machine is 90% back to Catalina boot speed!. It is a tiny bit slower to the first part of the slider bar showing, but literally only a second from there to actually being booted. Hurrah. Hope other people find the same improvement.