Originally Posted by dude_109
The pitch of the prop determines the speed.
Pitch speed is defined as pitch X rpm, and is the airspeed at which thrust drops to zero.
So all else being equal, more pitch and higher RPM are interchangable. (getting both is, of course, better than either one alone)
If the prop tips approach the speed of sound, or the prop blades flex so their pitch is reduced, then all else is not equal, but AFAIK that's about it.
Drag increases with speed squared, thrust does not increase with pitch speed squared. I suspect you're just finding the point of diminishing returns or bumping up against motor torque limits