An Experiment

First, 231 student participants were assigned to watch two YouTube videos at normal speed, 1.5x speed, 2x speed or 2.5x speed. They were told to watch the videos in full screen and not to pause them or take any notes.

After each video, the students took comprehension tests, also repeated a week later. The results were clear: the 1.5x and 2x groups did just as well on the tests as the normal speed group, on both tests. Only at 2.5x was learning impaired.

It seems, then, that as long as the material can still be accurately perceived and comprehended, its okay to speed up playback.