I too have this problem since setting up both my boards, Tyro 5 bottom and SD7 on top. I thought this was a problem unique to me as I have been diagnosed with Peripheral Polyneuropathy which does make my arms and legs fatigue very quickly.
I find that if playing the upper board for a roughly half to three quarters of a standard length song my hands tend to hang onto the keys rather than press then release but having seen this thread then maybe it is not my condition but age which is the problem.

Alan.