These days I type at about 80-90wpm which is not as fast as I once could (there was a point where I could achieve 130wpm on Dvorak but I was both younger and had less of a life). As is, though, it's very noticeable that I can type faster than I can actually compose text. I've also found that typing very fast tends to lead to a much higher rate of what I call "wordos," in which you mistype by completely swapping a word with another or omitting a word... these feel like symptoms of a sort of "buffer underrun" when typing that doesn't get noticed because I'm already having to move on to composing the next sentence.
So yes, I think there's a significant effect of diminishing returns on very high typing speeds, and I'd take a guess that it starts really kicking in at something not that high like 60wpm.
So yes, I think there's a significant effect of diminishing returns on very high typing speeds, and I'd take a guess that it starts really kicking in at something not that high like 60wpm.