As you said, now that playback devices appropriately adjust the frequency so that people don’t sound like chipmunks, my daughter and I find it very easy and effective to listen to most of our Youtube videos at 2x speed, by default.

Similar to speed-reading, I have found that it is optimal for podcasts and videos that are take a conversational tone. As you suggest, it is surprising how slow the information rate is for such communication styles!

However, of interest is that like speed-reading I have found that it is often counter-productive in presentations that are more formal and educational. For these materials, they should probably be played at 75% speed of original speed. This is in part, because this kind of material is already typically information dense, but also because the subtleties of material are often easily missed on first pass, and ultimately require readers to go back over same material thus nullifying the advantage of speaking quickly.

It is interesting to note that in recent years, many of the successful authors of reference material have picked up on this realization, and so instead of reducing the information rate by explicitly talking a bit slower, the trend is to make educational materials more conversational in style.