After trying everything, the solution to the problem was to clean install and don't mess with Time Machine.
Big Sur messes up with the profile. So if you clean install and restore the profile, you restore also the slowness together with it. The best way is to just copy-paste all your files from a backup and install the apps from scratch. I did this and now my 27" 2017 1TB fusion iMac boots in 30 seconds!
jimdem
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This is completely unacceptable. I have the exact same issue with slow boot times on iMac 2017 1TB fusion. You pay for an expensive machine and you have the boot times of a 15 year old PC. It is supposed to "just work" without needing "hacking" with workarounds etc. These are professional tools. Horrible.