I'm with Lawrence on this one. I've read interviews in Acoustic Guitar magazine, and others, where the musicians always encourage slow playing. I had a friend who didn't know squat about music theory or scales, and he could play Van Halen's "Runnin with the Devil" perfect, only because he got the tab from a guitar magazine, learned the chords, and wore out his cassette player breaking the song down. I asked him how he could emulate EVH's playing, and he said break it down, and slow it down. He even learned EVH's wild lead in Atomic Punk with all that crazy finger tapping.
I think sometimes we are so engrossed about learning to play a tune (or anything) that we forget we have to crawl before we run. And rightfully so, to do otherwise doesn't serve a piece justice.
This is why i appreciate Dave's style of teaching, it's about accuracy and not speed, especially with these bluegrass tunes, which we all know are played at smokin' speeds often above 120 BPM.