A while back, I was showing symptoms. It turns out, that I was tensing during periods of waiting to see feedback. Now, being aware of it, it's no big deal, and I don't do it anymore. Symptoms gone.
Are you sure you're typing 20 letters per second?
In game terms, input can approach that speed, which is really what I was writing to. Should have said, by analogy.
For me, a whole lot depends. If I am transcribing, I look at the screen, and it can matter. Same for a quick type of a common phrase. I'll blast that out.
For most things, it's a few CPS and slower.
And this is all modal. Interactive, like cursor navigation is different from general character input, which is different from common, short words.
They may just do it when the latency varies.
However, for a string of them? Yeah, could be a factor. My own trigger was mere awareness.
Just knowing it couldbe latent to excess triggered that tensing, like "stay ready."
All I'm saying is its plausible. People can, do, will respond very differently to these kinds of things. Worth investigation.
For i.e. the arrow keys, or anything interactive you will probably have to be watching for the response.