The speed is limited by rpm of the rotor head. As you go faster in a heli the blade moving into the direction of motion will experience a higher airspeed compared to a hover and the one moving away from the direction of motion will experience a lower airspeed compared to a hover. As you go faster the blade moving away will eventually have so little airspeed that it is essentially stalled. This then causes the helicopter to pitch nose up due to gyroscopic precession and slow down. This phenomenon is called retreating blade stall and you can reduce this by turning up the headspeed.

 

If you are after a pure limiting factor, I would say the power plant.

To simplify it, even at the risk of being inaccurate,

When you get lift, which is what the rotor blade makes, you also get drag.

The faster the blade moves, the more drag you get, as well as having less thrust available.

The more drag you get, the more power the engine needs to make to over come it.

 

Interesting sound.
(Eeeeeeeeeeeeeeesrrrrrrrrroooooooowwww-WHACK!!!)

 

But does the sound barrier limit how fast you can spin the rotor head? If you can spin 700 size heli to 3,400 rpm to offset the stall on the lagging blade in order to travel at a faster speed, then as leading blade travels faster would the blade tip be approaching the speed of sound (1234 km/h) What then? Can the rotor tip break this limit without issues?

 

The jet stream sound of that heli was really frekin nice... so officially the world speed record is 311 Kph