Springs weaken from cycling -- not being held in tension.
Was wondering how long that would take . . .
A new shooter friend of mine has been running some rounds through my Xdm.40, and though, initially, it was hard for her to rack the slide, she commented recently how it's now gotten much easier.
- - Side note - - - although a petite woman, she has no issues with recoil management and is seriously considering getting her own now!
I'm fairly certain there hasn't been that kind of decrease in strength of the spring in the last month or so, and I'd sure hope there won't be that much over the next few thousand (or more) rounds, or I'll be buying a case of springs. But I am certain that her technique has improved, and she's remarked that though it's still tricky, she feels she's "getting better at it."
No doubt there are differences in spring strength between different guns and calibers, necessary for proper function. There may or may not be some weakening of an individual spring that comes with use over time; if so, I'd lay money it's fairly insignificant, but I have no hard fact research to support this.

More significantly, over several hundred (or thousand) rounds worth of loading, dryfiring, malfunction drills, and etc. of racking the slide, maybe we just "get better at it."