For maximum efficiency, exhaust velocity should closely match vehicle speed. That way, most of the energy in the fuel goes into the KE of the craft and not into the exhaust plume.
The Roton was a rocket-helicopter during the early stages of its flight; tip-jets spinning the rotor. A large amount of atmospheric air would be forced backwards at relatively low velocity. High jet velocities conserve reaction mass but waste energy.
Basically, a good idea but they never made it work and the company went bust