Slow is perhaps the hardest thing and getting harder with every passing nanosecond.
Everything in our culture is moving fast and faster. Electronic signals, multitasking, so much to do, so little time. And it shows up in our musical lives.
The hardest thing about teaching is learning to listen because it demands the ability to distinguish between one's own trained musical speed and the speed of the student, which is usually exponentially slower--even the so-called advanced student is slower than the practiced pro.
My own playing is much improved as a result of learning to listen slow.