The rotor RPM in a steady autorotation won't change much from zero to Vne (autorotation), after all the transient effects die down, and the collective is fully down (or in the same position).
At least that's my experience from doing lots of autos in the Bell 206 series.
Above Vne (autorotation), the rotor RPM can decay pretty quickly, which is why it's the Vne(autorotation).
The rate of descent increases any time you go faster or slower than the Velocity for Minimum Rate of Descent (VsubY) - that should be obvious when you think what a graph of Rate of Descent vs. Airspeed is all about.
But the rotor RPM doesn't change.
The reason the Rate of Descent increases away from VsubY is that going has more fuselage drag (pretty obvious) When you go more slowly, the reasons become too complicated for me to remember or care about, so I give up and just show that the Rate of Descent increases....