check out this thread: https://developer.apple.com/forums/thread/666661
It's clearly a big flaw in Big Sur.
I'm using a MacBook Pro (Retina, 15-inch, Mid 2015), worked splendidly on 10.14
It now feels like a cheap supermarket promotion with a version of Windows that is far too heavy for it....
I hope Apple will see the thread growing and patch this huge issue.