I have trouble. When I try to play YouTube videos, they play super fast. I'd include a video of the problem but it would be too large. Suffice it to say that a 24 minute video finishes in less than 5 seconds when I hit the Play button. This is new with Big Sur. I just installed beta 9 last night. I have turned off all extensions, deleted my history, restarted Safari, all to no avail. The problem happens with Chrome too so might be a QuickTime problem or something underlying.
This is a commonly reported bug that occurs 'sometimes'. Another solution, short of rebooting, is to kill the coreaudiod process.
You can do this in Activity Monitor, or in the Terminal with
sudo killall coreaudiod