Thanks for confirmation. Hmm, VLC and mpv were lagging even more? That is strange a bit. Lagging of such video in every resolution in Qubes OS is expected on any hardware, but I thought Firefox+html5 is slower and less effective than VLC/mpv players.
It is not about codes, because better CPU would solve it (but it doesn’t), it is about painting/drawing decoded raw frames in X11 under Xen.
Movies or TV shows do not often have such dynamic long smooth panning that would make us notice these inevitable lags. I watch movies on Qubes OS with not real issues, too, but I am sure lags are there, and playing the same movie on bare GNU+Linux is really much more fluent and smooth, but I just got used to micro-lags of Qubes OS.