Safe mode fixed Big Sur?

A couple of weeks ago I decided I would finally move my 2013 MacBook Pro from Catalina to Big Sur (can't go to Monterey). I immediately noticed performance issues, especially in Mail. Obviously if I had searched the forums I would never have done it, but the deed was done and I dreaded the idea of going back. Searched the forums here for hints at a solution. Tried a few things to no avail, then saw a comment from a member who said that he noticed a massive improvement after his Safe Mode test. That's all he did - start in Safe Mode. Of course I thought there was more to it, but it was an easy thing to do. Well, lo and behold - it really worked! Mail was a particular dog and it's working fine now.

So why did a restart in Safe Mode fix Big Sur performance?