The issue is likely that in Qubes OS you don't have hardware acceleration. The bottleneck might very well be single thread performance, though your tips are worth a try.
For me it depends on the specifics of the video, usually ~10 Mbit/s 1080p x264 and YouTube at 1080p is mostly ok, but I don't even need to try 2160p content. (i5-9500T)
Works fine for me. Did you try alt+F11 and giving enough RAM and CPUs?