1219 is a record as far as I know. We measured it explicitly by getting the screen reader to read a passage and dividing. I spent months working up from 800 to do it and lost the skill once I stopped (there was a marked level of decreased comprehension post 1000, but I was able to program there; still, in the end, not worth it). When I try to described the required mental state it comes out very much like I'm on drugs. Most of us who reach 800 or so stay there, though not always that fast for i.e. pleasure reading (I do novels at about 400). it's built up slowly over time, either more or less explicitly. I did it because I was in high school doing muds and got tired of not being able to keep up; it took about 6-8 months of committing to turn the synth faster once a week no matter what, keeping it there and dealing with a day or two of mild headaches. Note that for most blind people these days, total synthesis time per day is around 10+ hours; this stuff replaces the pencil, the novel, etc. Others just seem to naturally do it. You have little choice, it's effectively a 1 dimensional interface, so from time to time you find a reason to bump the knob. And that's enough.
Whether and how much the skill transfers to normal human speech, or even between synths, is person-specific. I can't do Youtube at much beyond 2x. Others can. It's definitely a learned skill.
0: https://ahicks.io/posts/April%202017/rust-struct-field-reord...