Despite the age, if the gun hasn't been fired (or cycled) much, it could still have "new" stiffness".
Also, check for any burrs or tool marks on the face of the hammer and on the slide where they rub together. Rough surfaces can make it harder, until they polish from either wear or deliberate work on them.
If you want to know if its the bushing, simply replace the bushing with a solid one, and see what it feels like. It could be any one of the mentioned things, or even all in combination.
My guess would be its still stiff because its not "broken in" but I have no way to know from here.