I used to use one of those speed reader apps. I found that if I set it at ridiculous speed - 10,000 wpm! - you can still read the words but not really "read" anything i.e. be able to say anything intelligent about what you just "read."

But then I would shift back to 4,000 wpm and suddenly it seemed so easy to keep pace. And the longer you stayed at 10,000 wpm the easier 4,000 became.

I feel our brains are totally up the challenge.

I've listened to a number of fiction audiobooks at 3x. I find it takes a couple of minutes to adapt to a new narrator/speaker at 3x, but it's not really a big thing.

I'm not sure I'd retain much from a technical conversation at 3x -- for me the drop seems to be somewhere between 2x and 3x.

I read a lot and my pace naturally slows down or speeds up depending on the density and difficulty of the material; I'm sure a similar effect must apply to the speeds you need to follow a lecture or podcast at to actually understand it. I can't read a book-length classical epic poem or a dense technical book at the same kind of speed I can comfortably read a Sherlock Holmes book (well, I can, but I won't retain anything).