With cold air only "reversed" probably a lot less than 50%. The major retardation work of reverse thrust is that done by reducing the velocity of the engine mass flow to about zero wrt the aircraft itself. So a (really) simple analogy, ignoring the hot gasses, is of each engine being similar to a drag chute but with, about, a constant mass flow. Hence, like a drag chute, the retardation effect reduces with speed.