Back when I was practicing Zimmerman bowing exercises on the double bass, my teacher revealed a mental barrier I had erected for myself. Each Zimmerman exercise had a written metronome marking, typically in the range of quarter note = 120 to 132, and the exercises were mostly sixteenth notes (four notes per click).
At the time, the marked tempos would seem impossibly fast. I would work on the exercises slowly and gradually increase the tempo, but I would never be able to acheive the marked tempo because in my mind, I had already decided that it would be impossible.
Then one time at a lesson, my teacher told me to turn off the metronome and just play the exercise as fast as I could. I immediately was able to do it considerably faster that the marked tempo, because I wasn’t looking down at the metronome thinking about how difficult the tempo was supposed to be. It was an important lesson which I still think about all the time.
Today as I practice the organ, I work out exercises slowly in order to get the fingerings correct. The I gradually increase the speed to my ‘breaking point’, the tempo at which the exercise starts to fall apart. At that point, I will remind myself that the metronome marking is an illusion – not an upper limit.
Then after a few minutes of cleaning up the passage at my breaking point tempo, I usually find that I’m immediately able to play the passage much faster.
At least on my good days…