They don't..........unless they are lighter than what they are replacing. Use of them is usually dictated by things like cam choice, milling, etc., with no thought as to the weight. Pushrods are part of the weight of the valvetrain, lightening them will allow for less weight that the valve springs need to overcome to return the valve to its seat. Lifters, pushrods, a portion of the rocker arms, top collar of the valve spring are parts of the moving valvetrain. Lightening as many as possible will yield an engine that will spin-up quicker.