I have a Glock 17 - the stock recoil spring is not particularly stiff for a semi-auto. Are you sure the previous owner did not install a heavier recoil spring? That would make a huge difference in perceived effort to rack the slide.
What I would do before you do anything else is go to a gun store and try out a new Glock 19. If you can rack the slide on a new one, that would be a strong indicator that the Officer who owned the gun put in a heavy recoil spring. Then, if this is the case, it's a pretty easy fix.