If you go around the rim at a constant speed with a faster grinder it'll be a worse grind than with a slower grinder. If you think about it the thing about the grind that makes the brake work better the is the way you put little tiny divots in the rim levaving a sharp edge. If you have a fast grinder then it will hit the rim more times per second meaning that the sharp edge's of the divot will be a lot closer together. Its like a bed of nails, If you have the nails really close together(fast grind) so it is effectively a flat surface, you can lie on it no problems, yet if you have say 10 evenly spaced out nails(slow grind, yes I know a bit of an exageration)the it will hurt. It's all because of pressure. So in conclusion. Fast grinder=move around the rim fast slow grinder=move around the rim slower I would say fast is about 13k rpm and slow is about 10-11k rpm