To reply to some of the questions of the thread: Yes most of the user having slow down have upgraded from Catalina, but that is exactly what is expected from a mac. My fusion drive is still properly configured and recognized by the system. A clean install without time machine is not a solution but rather a workaround: i have many customisation to apps and system i hardly remember how i did them; plus i have one year worth of time machine backup I would like to keep. Beside that I keep my user profile very clean, I manually purge old files from time to time I honestly cannot believe my settings can be the reason for that slowness. I guess it has more to do with what is in the system folder, but that is out of my control and it is apple duty to keep it clean and lean. Big sur might be intentionally crippled on intel mac to move users to new Apple Silicon based macs, but i really hope that is not the case. I have been updating my mac yearly since 2015 with new macOs's and Big Sur is the worst update so far in terms of performance drop. I highly suggest against updating to Big Sur: if you're happy with Catalina, keep it or you will be disappointed.

iMac 2015 with Fusion drive: after installing Big Sur 11.0.1, startup is painfully slow. At least three to four times slower than 10.15. It seems as if it is reading everything from the rotating HDD instead of the SDD portion. After boot is complete, apps are then pretty snap and speed is back to normal.