Why have helicopter top speeds not increased much over time?
The underlying limit is that the forward-moving blade tip must remain subsonic, and the receding blade must not stall.  Hybrids do better than conventional or compound helicopters because the rotor can be slowed when the fixed wing picks up some of the lift load -- but this only solves the forward blade tip speed issue; unless the rotor can be operated at zero lift, receding blade stall is still a problem.

Some futuristic designs have proposed completely halting the rotor in forward flight (see the aircraft in the movie The Sixth Day with Arnold Schwarzenegger -- cheesy movie, but the aircraft were based on a real concept), using an airfoil shape for the rotor blades that can develop lift with either edge forward.  Others have proposed completely folding the rotor once forward speed is high enough for the fixed wing to support the craft -- but as far as I know, no flying example of either of these concepts has been built.  Until they are, the same speed limitations apply to helicopters and autogyros that were discovered in the 1930s, only partially mitigated by hybrid configurations.

Bottom line is, a helicopter's niche is in going slow, not going fast.  Other VTOL solutions exist for high speed (thrust-hover jets like the Super Harrier and F35 are supersonic).  Attack and high-rate maneuverability in hover are the special domain of choppers; high forward speed is not.