No, I do not think bitrate or codec matter in this case at all. As I said, you can watch train-city-panning video in 480p and it will lag in Qubes OS.
How can it bitrate of this 480p youtube video be relevant if 1080p video works the same? The bitrate of youtube video a 480p is like 10 times lower.
So, once again, it is NOT about bitrate, not about CPU, not about RAM. It is about Qubes OS not able to refresh the whole screen properly without lags. So, display resolution is more important than video bitrate and etc.