I did a search on this and while similar issues have been discussed, I did not find one that directly hits this problem. I can play old time fiddle tunes I know well at fast tempos accuarately and cleanly, but what happens after about a minute at warp speed is I get painful fatigue in my picking wrist. It is not a sharp pain, but just a dull fatigue kind of pain. I usually have to switch to rhythm for a couple of go-arounds.
I am an intermediate player of a about 15 years. I play a fair amount, but not hours and hours a day, so I would not think there is an overuse issue. I don't post my fingers or my wrist. I broke myself of that years ago. I keep a loose pick grip and generally my arm is relaxed, but the wrist seems to tense up when the speed goes up.
I have heard the analogies of "the wrist should be like a dish rag" and "the wrist motion should be like shaking out a match." That is exactly what I would like to go for, but I am having a hard time getting there. Does anyone have any tips/drills I can incorporate into my practice for improving this?