Are you saying the slide stop is tough to disengage? It is obviously a separate issue from the recoil spring assembly. Or both. Since you bought it used, hard to tell what the seller might have done.
Why is it "obviously a separate issue from the recoil spring assembly"? Perhaps my description of the issue/problem hasn't been clear enough?
The slide release moves easily for the first portion of it's arc, but when it's pressed farther, to where the release needs to drop below the notch on the slide, it becomes very difficult to move. Pulling back on the slide releases the pressure and allows the release to continue down. There's nothing obvious inside the gun, causing this; with the slide off, the lever moves easily.
If the slide is locked back, and a heavy recoil spring is in use -- as was the case originally with the X-FIVE --
that recoil spring is tightly compressed and must be compressed just a bit more to release the slide. Doing that is difficult is you just use the slide stop to do it. It's like the recoil spring is simply too long (and possibly stacked.) I get the sense that were the recoil spring a half-coil shorter, most of the problem would go away, but I'm not going there. I didn't compare recoil springs lengths when the new one arrived, knowing you can't compare old and new unless they're in the same working state. (Recoil springs take a set -- shorten a bit -- with use.)
As noted, installing a lighter spring made it much easier to release the slide stop -- and made it almost manageable. If it was a separate issue from the recoil spring assembly, I don't understand why changing out the spring would have made such a marked difference. It did.
I have had other P-226s, but this is my first X-Five. As I'm sure you understand, the recoil assembly on the X-Five is substantially different than the standard P-226 -- and has a surprising number of parts!
I also have a P220 Super Match which is one of the single action only models of the 220, and a P228 reworked by Gray Guns. The slide stop on the Super Match is slightly easier to release than the X-Five (with the new, lighter recoil spring installed), but like the X-Five, not easily depressed by the strong hand thumb. The slide on the P228, which is very low-mileage gun, can be easily depressed using my strong hand thumb without having to shift my grip to get better leverage, etc.