I can watch YouTube content at 3x without issue. I did this without much intentional effort - I simply downloaded an extension which allows me to speed up the video in increments of 0.1x using a keyboard shortcut. Whenever it felt slow I would speed it up, and whenever it felt too fast I would slow it down. Without paying much attention to the actual numbers I had reached over 3x within a month or so.
I tried this right now with the trick below. 2x was no issue but 3x... that was a big step. It sounded like word salad. As if my brain was decoding the words out of order and was unable to assemble them into sentences.
It also depends on the density of the original content. Some things flow nicely at 1.5; others can be cranked up to 3x or even beyond if you turn on subtitles because they're just another YouTube video padded to hit the magic 10 minute mark, or whatever it is now, and you don't really need the audio.
I do it via `document.getElementsByTagName("video")[0].playbackRate = 3.0` in the dev console. You can up arrow to it similar to a normal terminal on the next video. The nice thing about this is you can put any float in e.g. 3.333.
The extension he probably used is Video Speed Controller; it works on any HTML5 video, and you're not limited by whatever UI they've wrapped around that built-in functionality.
Streaming and download/playback tools can offer greater flexibility. I typically use mpv, though other options are mps-youtube, or for download + playback, youtube-dl and xine, mplayer, mpv, mps, or VLC.
I find I can typically listen at 1.25 - 1.75 readily. Exceptionally poor content I'll bump to 2x. I don't generally go much above that unless I'm fast-forward scanning video for specific content.
I created a simple browser extension for this purpose, "YouTube Turbo Button". As mentioned in the other comment, it can be easily done through the console, but I found having a button for it more convenient.
