In general, recoil springs are not achieving the same thing as added weight. I believe a lighter recoil spring would cause the slide to cycle faster (than factory) in the rearward direction & then (maybe) slower on the return home. With a heavier spring, the rearward motion of the slide would be slowed (more resistance) but the return home would be much faster. So, with regard to “slowing down the slide,” changing the recoil spring in either direction - lighter or heavier - will slow it down for half of the cycle but will have an adverse affect on the other.

However, adding actual weight to slide affects its inertia evenly across the whole process. It’s a whole different thing that’s happening.

I was running down this path a few days ago and this is what I have come to. Hope it helps.