I just replaced a 2013 iMac running Catalina with a late 2019 iMac and installed macOS Big Sur. It is unbelievably slow. Boot time painful but that's only once a day so I could live with that, but almost everything you do on the iMac is painfully slow - even loading System Preferences! Safari is the worst of all - pretty much unusable so I have switched to Chrome which is a little better. If the WiFi card hadn't failed on my 2013 iMac I would go back to it immediately. Hard to fathom how a 6+ year newer iMac can be so massively slower or how Apple could treat its loyal customers with such disrespect. Big Sur should be renamed Big Slow.