Neat, I hadn’t seen the data presented that way before. Your graph also shows an important difference between land and sea: because the sea surface is usually smoother than the land (no trees and hills), there’s less turbulent mixing and the boundary layer is usually thinner. So the neutral point you’re talking about is much lower at sea.

The weaker, thinner boundary layer also means the overall wind speed is much higher at sea, which is why people go to the trouble of building offshore wind turbines.