alt Hacker News> (it is the video itself not YouTube being a resource hog, I checked)
Did you check by downloading the video and playing it with a good standalone video player like mplayer, vlc, or mpv? If you didn't, then you didn't disentangle the web browser from the video playback.
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.