Same problem here on my 2017 27" iMac with 16GB RAM! It was snappy with everything I did on Catalina, but it is garbage now with the Big Sur update. Takes longer to boot, is laggy to browse the file explorer, and simply saving an image in my GIMP photo editing app- which was almost instantaneous before the update- now takes 30 seconds or so.
I really want to roll back to Catalina!
jm31828
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Bothell, WA, United States