For iMac users seeing a slow down with Big Sur that are *not* running Avast: Does your iMac have a fusion drive? (large spinning disk, small SSD cache 'in front' of it)
Did you upgrade from Catalina, from an older OS like Mojave, or did you perform a 'clean-install' of Big Sur by booting from a flash drive installer and reformatting the hard drive?
Is it possible that the fusion drive configuration has been messed up by the upgrade / reformat process?
If so, then your iMac could be effectively running directly from the hard drive alone - without the benefit of the SSD cache - and the fix would be to rebuild the fusion drive and reinstall from scratch.
jonathon-b
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An update to my earlier post (2015 iMac 27" with fusion drive): The following steps worked for me:
Boot with option key down into a Big Sur installer USB flash drive
From terminal, execute 'diskutil resetfusion' to wipe, reformat, and rebuild, the built in fusion drive
Return to main recovery menu and install Big Sur from the USB flash drive.
No noticeable slow down.