I noticed I had some greyed-out SOCK5 Proxy Server details input on the Connections tab from an old VPN service I had. Even though I had Type set to (None), I wondered if this might still be implemented and be cause for the strange occurrences since Big Sur installation so I chose Type: SOCKS5 again, deleted the details, unchecked all the Proxy Server checkboxes, set Type back to (None), and pressed ok. IMMEDIATELY my torrents jumped to life.
It seems like the Big Sur issue isn't with Big Sur as much as it is with QBittorrent reading either old preferences or not reading them and using some pre-coded ones. Maybe this is why when we jiggle or modify some of the settings, it then gives its head a shake and smartens up.
Hoping this might help others… as going back to 4.1.9. (as has worked for others) did nothing for my speed and actually corrupted on of my shares. Live and learn.