One of My more vexing struggles concerns the subject of speed. I seem to regularly hit the wall when trying to do something fast. It can be a reverse punch or an arpeggio; either way, I get to a point where it just doesn't seem to get any faster.
But now, things are finally starting to change. The problem is both physiological and psychological.
I got the first clue a few months ago, in karate class: Speed requires relaxation. It isn't easy to get an arm or leg moving, then relax it, then tighten it up again at the point of kime; but subsequent developments indicate that yes, it is actually possible. The moment I stopped fighting my own punches, as it were, they did move distinctly faster than before.
My clarinet teacher has been helping Me deal with the psychological aspects of the speed problem. At its heart, this is a matter of trust. After playing a wide variety of material over a period of nearly four years, My hands do, in fact, know where all the notes are. My conscious mind, on the other hand, isn't quite sure about that; and it keeps interrupting.
So that's what I'm working on at the moment: Dealing with the mental block caused by inappropriate attention and unwanted dialogue. One thing that seems to be helping is practicing in strict rhythm, with a metronome. While the rhythmic practice builds up more precisely-triggered muscle memory in My hands, the ticking metronome gives My conscious mind something to keep it occupied and out of the way.
And, after I've got the rhythm working a bit more reliably, it'll be time to concentrate again on relaxation. All in all, an interesting problem.