Storing the gun with the spring compressed does not soften the spring which is a good thing. I keep all of my semi auto guns cased and stored in the open and locked back position for safety reasons just as a habit. Springs do not lose their spring property when left compressed or relaxed. They wear out from cycling.
Your Shield has a tight spring in order to protect the frame from being battered by the short recoil of the smaller slide. You could go get a lighter spring, be happy with racking the gun and buy a new frame (gun) in a few thousand rounds or simply accept that a small gun will have a stiff spring.
Are your Sig/Baretta's subcompact CCW guns with 3" barrels? If they are and they are easier to rack, you've found superior guns.
The suggestion of using an empty mag to lock the slide back is a good one. Another suggestion is to hold the slide with your left hand fixed and ram the gun forward with your right hand on the grip rather than trying to pull the slide back with your left hand and holding the gun fixed with your right. That's assuming you are right handed and your right hand is dominant/stronger.
If you go in with the mindset that this slide is going to be stiff and not like a smooth big full size slide, it seems easier to overcome. At least that's how I approach it.
Enjoy your Shield. But just remember it is a small CCW defense gun made to save your life and not primarily a range toy (although some have fun using it that way).
