I've spent a lot of time in my career working on weird rendering issues on websites/devices. Believe me when I said "I checked", I know WTF I am talking about.
The only thing that was different was Wayland vs X11. Same browser, same browser settings, same OS and same plugins.
> Same browser, same browser settings, same OS and same plugins.
Neat. Did you test outside of the browser? Based on your report, it sounds like you didn't. As you must know, the renderers in web browsers are very, very complex. I suggest you test with a standalone video player before you go blaming the underlying windowing system for performance issues.