The ear is many times faster than any other perception system--in fact, hundreds of times faster. I think of musicians as people who aspire to that almost immeasurable rate of speed. Musical children experience music at that speed. Everything in my experience supports this observation.

My question: Why is so much time spent slowing their ear responses down to the level at which they can control their motor function? Shouldn't it be the other way around? Shouldn't the motor function be left free to respond at the -- here we go! -- speed of sound?

This observation applies mostly to learning at the piano. It's completely different with stringed instruments which come in child sizes, and where the feedback in sound is directly connected to motor skills--a whole different ball of wax.