- Edited
iternabe practicing at tempo well below one's own threshold does nothing to help gaining speed
I am also in the disagree camp, here. My current piece is a slow Blues and I practiced it very slowly. I got faster and faster from alone. I even did not want to speed up. Now it is too fast and I have to concentrate to keep my tempo slow during playing.
Edit: this is when practicing without metronome. With metronome you have indeed you increase the speed from time to time. But only when you can play it errorfree enough.