The rotor speed is determined by the conflicting needs of several systems:
For high speed cruise, especially at high altitude, the rotor is closer to stall, and so must have the max rpm possible. This maximizes the maneuver thrust of the rotor, should it be called for.
At all other times, the rotor has "too much" reserve thrust, and is operating at a point where it has too much speed and too little angle of attack to be at peak efficiency.
This is especially evident at a hover, where the rotor wants to be near peak lift and peak angle of attack for highest efficiency, and it is still useful up to best range speed, where the lower power required makes less fuel burn and more range.
The problem with using lower rpm in a hover is that the tail rotor is also slowed down, so that the max left pedal available is reduced, and even at lower altitudes, at high weights this pedal might be needed. Other than the tail rotor effect, hover generally favors lower rpm.
All this is basic helo aero, the flight manual has the procedures that you must use!