I took lessons from Jon Bjork for a couple years and he totally changed the way I practice. He had me find my fastest speed on a run where I was still playing cleanly. He then told me just to play it only one time a day for a week at like 50bpm. The following week we tested my top speed and it went up a full 20bpm. Surprised the fuck out of me. Seriously 1 time a day at 50 bpm is all it took. Before that I was always practicing stuff at my top speed. Or 80 percent then bumping it up 5% every so often. I made more improvement with playing slower in one week than I made weeks with my previous method.
This, chunking, and pick slanting has pushed me past what I thought was possible. I hit a plateau around 19 or 20 and just felt like this is as far as my talent will take me. God I wish I would have found a good teacher sooner