Ugh, I forgot and I just read it… where?
In Peak there is an example of typists doing sessions that are 10-20% faster, and then going back to normal speed once their concentration lags. (page 163)
No mention of it here. https://en.wikipedia.org/wiki/Flow_(psychology).
I would guess that we can’t totally generalize across all these activities. I mean when comparing across typists vs. violinists what would x% harder mean exactly? Whatever rule of thumb you find is going to be a guess when it comes to memory, so until someone does a huge research study on training memory champs…
So, why not feel it out? How much faster can you go and maintain concentration and the right mind-state without getting “swamped”? You could sort of ratchet your speed up every couple of days, and when it stops working back off a tad bit.