We've discussed before how the forward speed of helicopters is limited by the speed of sound and by a phenomenon called "retreating blade stall".
As the rotor approaches Mach I a shock wave begins to build in front of it and drag increases exponentially.
The relative wind flowing across the advancing rotor blade is the sum of the forward blade speed and the speed of the aircraft, while the speed of the relative wind on the retreating blade is reduced because it is running away from the forward speed of the aircraft. Therefore, the retreating rotor blade has to have a higher angle of attack in order to produce the same lift as the advancing blade. I'm still amazed helicopters even fly.
So the speed of sound is a physical limitation for true helicopters and in order for rotary wing aircraft to go faster, we'll have to explore technologies that make the aircraft something other than a true helicopter.
The V-22 Osprey is an expensive, complicated (and controversial) way to attack the problem.
Compound helicopters are another, less complex way to increase forward speed...
By adding a wing, the angle of attack on the rotor can be reduced because the wing is assisting in production of lift. The speed where "retreating blade stall" is encountered is thereby increased, which allows the aircraft to fly faster before encountering that problem.
This video shows flight testing on such a machine...
You can see the stub wings on the fuselage, and the ducted fan and rudder-like device that replaces the tail rotor. It's an exciting concept, and may help bump up the speed at which rotorcraft can fly before encountering aerodynamic limits.
(Hat tip to friend Terry for the link!)