I remember reading a study a while back about learning from videos at 1.5x to 2x speed. The key observation that actually made the learning more efficacious was to watch the video at 2x speed two times back to back. Apparently, this method was also better than watching the video one time at normal speed. I suppose it's because one get twice the exposure for the same amount of time.

I haven't looked for the study, but I can if you are truly curious.