My “flow” piano speed is 13.5 notes per second (270 bpm in triplets), doing arpeggios with the right hand. It’s where my muscles and brain “max” and I still have complete control. I don’t think I can go faster.
It’s a high feeling and I can stay at it for hours.
But different notes would be distinguishable. I’ll have to look at fast guitar/piano players doing single notes and see if they perceptually blend.