Helicopter main rotors are effectively a propellor flying sideways and suffer the same problem of transonics at high cruise speeds. The advancing blade's "forward flying" airspeed is added to the blade's rotational airspeed.
Some main rotor blades have swept tips, to delay those bad high speed effects.
Trouble is, as soon as the blade "turns the corner" and becomes a "retreating" blade, the forward airspeed steals the rotational airspeed, bringing the blade close to the stall. In fact the inner sections of the blade do stall at high IAS.
These are the main limits to forward airspeed of a helicopter and one of the reasons that the tilt-rotor was designed.