When I was first learning piano (six years old) my teacher was insistent that any piece be learned at the speed at which no mistakes were made, Didn't matter if it took an hour to play the minute waltz; do it with no errors. Only then could you increase the tempo, and only to the point at which no mistakes crept in. Let me tell you, as a six-year old with the attention span of a squirrel on uppers, I about lost my mind. But when I was ten, and took up trombone, that same slo-mo procedure made it much easier to learn the instrument. Slow repetition, until I got the muscle movements correct and knew just how far the slide needed to go to get the correct note without having to think about that aspect of playing the piece. I could concentrate on the music itself, not the mechanics of producing the individual notes. I'm still a rookie ball pianist. Maybe Low-A. But the issue is physical, not mental. Short, stubby fingers do not lend themselves to the task. On trombone, I was AAA, maybe even AAAA. The technique just flat-out works.