and it took quite some time before I understood the need to work slowly.Slowing down the tempo helped me to control many things at the same time : notes, rythm, colour, memorization, and improved my learning rate a lot. I found out that speeding up after wasn't really a big problem once you get everything working at low speed.I had been misled into practicing too fast by a wrong interpretation of Chang's book, at a time when my professor did not care about telling me how to learn, so I tried to derive my own practice strategy from various readings (like Chang) and probably got some ideas wrong.
I don't think Chang says anything to imply you should simply charge through things and settle for sloppiness. It's about improving the feedback loop- not throwing out careful work.
I thought practice makes perfect but for me clearly that is not always the case.