Wouldn’t all that depend on resolution and bitrate? Using a contrieved example: my understanding of video encoding is that a completely monochrome video using a compressed codec could play nice even in 4K because very few changes would need to be decoded and applied between frames (corresponding to a low required bitrate).
Depending on how different videos are encoded, they may be more or less affected by Qubes OS’ CPU rendering limitations at different resolutions if the bitrates are different. What I’m saying is that unless I’m wrong, comparing different videos based on resolution alone is not really meaningful, and that might explain why some folks find everything terrible (presumably trying to watch videos with higher bitrates at a given resolution) and others find things okay even at higher resolutions (presumably watching videos with lower bitrate). Even assuming that their opinions about what is okay and what is not are somewhat consistent.