So far, no issues with Big Sur for me. I didn't update over the 10.15.7. Don't have any slow boot issues either.
I didn't get to boot once, but I understood why that was. it was because I had cleared NVRAM, and for some reason I had to enable locked CFG, then "attempt" to boot, and then go back to UEFI and disable locked CFG. Same had to be done, when I was on F8 UEFI fw, but then I had to do it manually via OpenCore, as the disable function of CFG Lock isn't in the UEFI GUI. But now I'm sticking with F9i.