I have 2 macs - Macbook Air 2015 and iMac 21.5 4K 2017. On the Macbook I had so much "others" files that I wasn't able to simply update the system to Big Sur, so I did a clean installation. On the iMac - I did a regular update. The older and slower Macbook Air performs perfectly with the clean installed Big Sur and the iMac is slow as f***. I have experienced this same problem with the update from mojave to catalina. So guys, it seems that the better option is to back up your data and just make a clean install. Its not that hard... For me as a software engineer(with all the additional software and sysmem tweeks I need for my job) it took me around 10h straight to get my system as it was. Sometimes its better to invest some good 10h of installing everything from scratch, rather than loosing time in working on slow computer.