Some possible reasons to grind slowly.

1) If you grind fast, it will heat up the rock and the grinding wont be as smooth. The heat could also change the chemistry of the rock. Also, going slow with smaller forces is less likely to destroy small features

2) Motors heat up more when you spin them fast. This is worse on mars because the atmosphere is 1% the pressure of earth. This mean things don't cool nearly as well. Motors, electronics, and other devices have to rely more on radiating the heat away instead of through convection. This heat from friction is also wasting battery energy doing nothing useful.

3) The faster you go, the worse the vibrations. This would probably shorten the life of the RAT, the arm, and maybe even the rover.

4) Spinning faster would require larger motors and larger power electronics. Why waste mass on that.