One of the very first things that I would check is the linkage geometry to make sure that the moment arm isn't too small at maximum deflection. Many times the servo or servos have plenty of power just off of neutral but run out of steam when the aileron is trying to get to full deflection at higher flying speeds. Try pulling into a vertical position and see if the roll rate increases as the speed bleeds off. If it stays the same or increases then the flight surface is forcing or stalling the servo to a position less than full deflection.
The solution is to either get the linkage in a position of better mechanical when the aileron is at full deflection, or use a higher torque servo.