I'm experiencing the same troubles as most of you described here in comments. At first I also thought it was my Wi-Fi router going crazy, but then I noticed it was actually the update to Big Sur from Catalina I made a few days ago. The Wi-Fi connectivity swings from full connection to no connection, also ping going high and back to low. When downloading movie torrent, the speed fluctuates much more than before. Hope it'll be back to normal with new update very soon!