There's a hard speed limit for helicopters. Like, a speed that helicopters physically can't exceed. Above about 400km/h, helicopter blades literally run out of lift and the bird falls out of the sky. It's due to a phenomenon called dissymmetry of lift, and this video explains exactly how it works.
400km/h roughly equates to 250mph -- that's the 250 you see in the video above -- and that's the speed where helicopter blades' coefficient of lift falls sharply off a cliff. When that happens, the spinning rotor blades create unbalanced lift on one side of the helicopter (where they advance) versus the other side (where they retreat) and everything gets very complicated for the pilot involved.
The retreating blade stall phenomenon can be addressed with dual-rotor designs in larger 'copters like the CH-47 Chinook, but then engine power becomes the main consideration in reaching top speeds -- and there's still a point where dissymmetry of lift makes helicopter blades just stop working. [YouTube]