Human speech averages 150 words/minute, but human thoughts run more like 400 words per minute. Steve Rousseau decided to try "podfasting" (listening to podcasts at faster-than-normal speed) at progressively higher speeds to see whether he could consume more of the internet-mattress-subsidized high-quality audio bubble as he could before that bubble burst.
The upshot was 1.25x speed improved the pacing of many podcasts, while 1.5x was "initially jarring" but quickly "felt natural." But 2x was where things started to falter, requiring "an additional level of focus" at the outset of each program to adjust to the accent and cadence of the speakers, and the programs had "less emotional resonance" with Rousseau's thoughts primarily occupied by comprehension, not empathy.
At 3x, things just melted down, becoming distracting and difficult to comprehend.
Much of the work on high-speed audio has been done by testing visually impaired people and their uses of text-to-speech user-interfaces. I once listened in on a totally blind friend's experience of navigating through a web-page, skipping over UI elements much more quickly than 3x, and it was both daunting and incredible.
I confess that I've done a little of this, but primarily with video, because the visual side provides context cues that might get lost in the sped-up audio, and because videos are much more prone to having ponderous, lengthy sequences ("More pauses than a Pinter play!" -MST3K) that are best skipped over (looking at you, Stranger Things Season 3!).
Feeling competent in my base-level comprehension at 2x, I crossed the threshold into 3x. It took every ounce of concentration to just register what was being said. After 20 minutes, my brain couldn't settle into the rhythm of the conversation. I sat there for an hour, with my eyes closed, hoping that my brain would eventually "click" like it did before, but it refused.
At previous speeds, the audio would initially sound like it was on fast-forward, because it was, but not so fast that it would sound humanly impossible. It was like listening to an anxious seventh-grader give a presentation, coherent, but rushed. At 3x speed, the audio entered an aural uncanny-valley, too fast to be considered human, but slow enough at points that you could make out a phrase here, a sentence there. It was distracting in ways that the slightly-less accelerated rates were not.
I Tried Listening to Podcasts at 3x and Broke My Brain [Steve Rousseau/Onezero]