How is the speed of a helicopter controlled? Does the pitch of the blade vary for speed while rotor blade moves at a constant speed as with turboprops? Or is there a throttle mechanism for varying the rotational speed of the rotor for forward airspeed?
A helicopter in principle has a constant speed rotor. In fact, the allowed RPM range is very narrow, somewhere between 90-110% nominal RPM. This RPM range is achieved either by a manual throttle or a governor. The blade pitch is directly controlled by the collective.
If a helicopter rotor were perpendicular to the forward speed, you would indeed need to keep increasing the blade pitch (with the collective). However, in a helicopter, the rotor is nearly aligned to the airflow. The vertical 'chunk' of air the rotor takes therefore does not increase that much with increasing speed. At the same time, due to the forward speed the helicopter gains translational lift, which would actually decrease the blade pitch required.
There are many fine articles existing that discuss how the throttle is used to input power into the rotor disc system to lift the helicopter, and how the rotor disc system is tilted to change the direction the helicopter flies. Putting more tilt into the forward direction makes the helicopter fly faster in that direction.
Optimum cruise speed is discussed in this paper, where knots of airspeed vs fuel needed is derived/plotted. I wouldn't consider fuel wasted if one wanted to go faster, any more than driving 75 mph vs 55 mph on the highway is wasting fuel. You just get to your destination faster. "Time is Money", yes? So what is worth more - an extra gallon or two of fuel/hour, maybe outrunning a storm in the process, or climbing above turbulent air, vs your time being spent travelling and doing productive work (or vacationing) at your destination? http://www.eaa1000.av.org/technicl/perfspds/perfspds.htm