I think what you're talking about is a compression problem. In simple scenes without much movement, a low bit-rate compression (like used here for the web) does ok, but when ALL the pixels start moving in every frame, it has to ditch some of that movement in order to stay under the bit-rate cap. More complex scenes require a higher bit-rate for adequate playback, i.e., in complex scenes forced to a lower bit-rate, you get lowered quality and motion artifacting that looks weird.

Couldn't have said it better... I wonder if there is a way to keyframe variable compression rates for select segments of a video?

I know that FCP has the ability to add markers to chosen points in your sequence or clip, which then in theory affect the resulting render, but I've never tried it out.