Here is my understanding… The rotor blade of a hovering helicopter is continuously encountering the turbulence and vortexes left behind from the blade in front of it. However, if a helicopter is moving forward, the ’fresh’ or ’clean’ air allows the rotor blade to more efficiently produce lift.
Hovering in a helicopter is actually the most power demanding operation it can do. The reason is pretty much stated above but there is another thing that is working against you when you are hovering, wing tip vortices. Even fixed wing aircraft have wing tip vortices, however, obviously they only move forward so it doesn't generally effect their performace.
With rotary-wing aircraft, you still create wing tip vortices, however when you are hovering or travelling beneath ETL(Effective Translational Lift), you are not outside of them. What happens in that case is you do not have the full rotor span creating lift for you. The "wing tips" and the "root" of the blade are cycling through "dirty air", or their own vortices. What is essentially happening is that the wing tips and the root of the rotor disc are stalled and the only portions of the blade creating lift is the middle portion. Because of this it, hovering is the most power intensive operation for helicopters.
In addition, there is the tail rotor RPM requirements and that puts stress on the engine that can result in Loss of Tailrotor Effectivness while in a hover and there is Settling with Power but I won't get into those unless someone wants to know.
