I think the main reason helicopters handle turbulence better is wing loading. Small fixed-wing have relatively low wing loading, and the gusts affect the aircraft more than those with high wing loading, such as helicopters. Fixed-wing with higher wing loading handle gusts better than those with lower wing loading. I think the main factor isn't the speed of the wing spinning, it's the size of the wing. Very small wings, like helicopter rotors, don't provide the lift a relatively larger wing, such as on a Cessna, does. Turbulence that would be very uncomfortable to fly through in a Cessna is barely noticeable in a helicopter, and turbulence that is uncomfortable in a helicopter would be very heavy in a Cessna.