I really do appreciate the concern.
What I've done a poor job of explaining in this thread is that audiobooks are ... recorded ... at ... molasses ... speed.
When I listen to a book at 1x, it sounds like they are doing a bad William Shatner impression.
If they didn't record them slowly, I wouldn't need to speed them up to a normal conversational pace.
I'm not a neuroscientist but listening at 2x taking twice the brain processing feels off to me. I suspect that it's sort of like variable bit rate encoding, and that factors like practice, narration quality, sound quality, external noise, other distractions all come into play.
When you're sitting at a table with multiple conversations going on around you, do you feel like your brain is working twice as hard to separate out what is being said to you? Our brains have evolved some pretty impressive special case handling features for these scenarios.
If the book is recorded at essentially half-speed, then yes fair point. :)
> listening at 2x taking twice the brain processing feels off to me
It's not about the listening -- you're right, that's all habitual in terms of converting sounds into words. It's about the cognitive processing of the content. Whether you're listening to fiction or non-fiction, you're engaging in twice as much imagination, world-building, conceptualization of what you're listening to in the same amount of time... therefore a much greater proportion of your total attention and focus is engaged in that, which leaves less for paying attention to the world around you. It's mathematically twice as much information streaming in, and our brain's attention mechanism is finite.
People aren't usually consciously aware of these things, in the ways they insist they can drive fine even though they're tired, even though they've had a few, etc. Because we literally can't notice what we're not noticing. But accident rates tell a different story.
You've made some well-argued points. Thanks for speaking up.