To build on what Graviman said and to speculate based on my knowledge of Dynamics, variable rotor speed is probably the best way to go for a high speed coaxial helicopter, but it is not without its difficulties.
If you want to run at one rotor speed for hover up to 250+ kts, the rotor speed will either be so slow that hover performance is lousy (max thrust roughly proportional to the square of RPM) or there will be extremely high drag on the advancing blades at high speed due to compressibility. Neither of these cases is satisfactory and using a high rotor speed in hover and lower in high speed flight solves both problems.
A rotor blade has natural frequencies in both bending directions (beamwise and chordwise), and torsion too. ALL of them change with rotor speed. The issue with vibration is that the blade are excited by airloads at harmonics of the rotor speed. If a rotor turns at 10 Hz (600 RPM) then the blades will be forced at 10 Hz, 20 Hz, 30Hz, 40Hz, etc. A rotor blade had better not have any natural frequencies at those multiples at its intended operating speed, especially if one wants to cut down on cabin vibration and increase blade service life. The big problem with variable rotor speed is that the blade natural frequencies have to be kept away from those multiples for ALL intended rotor speeds. Keeping them located correctly is hard enough at one rotor speed, let alone a range.