After replacing the cables and you still have a stiff throttle and none of the other suggestions improve the throttle stiffness, the throttle tube and cam gears being worn can cause the problem .
Quick way to tell if you have an upgraded cam assembly, with the upgraded cam, the hole that the cover goes into, is the same size all the way down, not a step like the one you have, so if you decide to replace the cam you will need an upgraded cover as well .