Does anyone know what actually causes the lag and freezing?

Not every video seems to have it, most primarily currently airing episodes seem to run smoothly, but other episodes usually from already completed series often seem to freeze frequently on many of the episodes. Does anyone else experience that issue where a video only loads up to a certain point, freezes, then you have to skip ahead a bit to get it to keep loading?

Is this an issue with the video itself? Or is it something else that causes it?