Where critical speed comes from
A ball pressed against the wall at the top of the drum stays there when the centripetal acceleration equals gravity. Solving that balance gives the critical speed in revolutions per minute as roughly 42.3 divided by the square root of the inside diameter in meters (a small correction for ball size is ignored here).
That is why big mills turn slowly. A 2 meter mill reaches critical near 30 rpm, while an 8 meter mill reaches it at about 15 rpm.
Try it: drag the speed share from 60 to 92 percent. The balls switch from rolling down a slope to flying off the shoulder, then pin to the shell and ride around with it.
A teaching model
The ball paths here are drawn, not simulated, and the drum turns slower than a real mill so you can follow it. Real operators also tune ball size, charge level and liner shape to get the best grind for the energy spent.