She IS pretty fast driving that scooter. That's why I think her "disability" is really the same as Fluttershy's "weak flying" (I use quotes because we see that Fluttershy CAN keep up with Rainbow Dash if you give her enough motivation, so I don't think there's anything wrong with the wings in a physical sense in both cases - there's another factor involved that explains what's going on).
Well, just because you can run fast, doesn't mean you can fly. I think it's something in the wing shape. You can put a propeller engine on a car and it will move fast, but if you don't attach the right wing shape/size onto the car, it won't fly. So, her wings can make the strong propulsion, but can't keep her in the air.