I know you don't want to hear what I'm about to say, but it worked for me.
I re-installed Big Sur by restarting the Mac and holding CMD R until the display shows a progress bar. Then follow the steps to simply re-install. It needs wifi. You may get an error about wifi, just goto the top right corner and choose your network.
My 2019 Mac Air 13" with ssd took 40 minutes to download (it only shows a progress bar but I'm sure it was downloading) and then it needed me to Power it on again. (just check on it, my display went to sleep)
After that it took about 35 minutes and now.... Mail worked within 3 minutes. I could search anything, anybox
It took just over an hour, and tah-dah.
Best Wishes, Warmest regards. (LOL)