The javascript engine and rendering engines are what are generally affected by being backgrounded. So yes, generally the media library will continue receiving the combined video/audio stream and not bother rendering the video frames
Presumably there is no easy way to disable page JS but keep your youtube video downloading.
If they don't render while backgrounded, how do they do that without being noticeable?
I couldn’t find info on how often a keyframe is expected from YouTube, but I’m recalling the bad old divx days where you sometimes had a frame per scene and skipping was very slow.