150 pounds of person presses down on the snow the same no matter what board profile is under their feet. In a camber board it's just spread out over a larger area.
I highly doubt that if you had two identical boards and riders, with one board being cambered and the other one being rockered, you'd get any real speed difference...
Flats are all about technique. Stay on an edge and you should actually be faster than if you're flatbasing. Also it might be a difference in your boards. If yours is older the structure might be gone. Might be time for a base grind?!?