Anyone speaking improvisationallly (not from a script) can be listened to at 1.5x at least. I get through a lot of tutorials at 2x.
I don't mind audio, but I definitely don't understand the value of "talking head" style videos. They're, if not the worst possible information density possible, pretty darn close. A couple hundred megabytes to see someone speaking a few dozen kilobytes of text, and I still can't search it or skim it to see if it covers what I want.
I actually find that, for talks with slides, I do like having the inset speaker video. What I tend to do is to start off with a full screen talking head intro with mostly the slides/multimedia and a small speaker video.
However, are there any good automated transcription tools that can make a good transcript from an imperfect YouTube video? Most of what I've seen is good for humor, and I've even seen some used in a legal deposition context that are downright dangerous (inserting most likely word or phrase in place of what was actually said, so actively decreasing and corrupting info content). This might not be as disastrous for a lecture where there are also office hours, but... anything good out there?
Automated? No. But if you're willing to pay $1/minute and the audio is good/accents aren't too heavy/etc. there are a bunch of options.
(And yes, I usually also go through presentations at 2x).