When I’m understimulated by a teacher, my mind wanders. I’ll start thinking about something else entirely. When my mind comes back, I’ll have missed a couple minutes of content. I can overcome this with willpower, but doing so is exhausting.

Playing videos at 1.5-2x fixes the problem entirely. My attention is held without any effort.

If a concept is new or difficult enough, I just slow the video back down or pause it whenever I need to think things through.

I don't think you can make blanket statements. Consider:

1. Teacher 1 speaks at 100 words per minute.

2. Teacher 2 speaks at 50 words per minute, for the exact same content as Teacher 1.

You're not really losing anything by playing Teacher 1 at normal speed vs Teacher 2 at 2x speed. For every learner, there will be an optimal teaching speed for a given topic. This may mean listening to some audio at normal speed, some at higher speeds, and some even at slower speeds. I always customise my playback speed to the specific content I'm trying to learn.