Newton's 2nd Law of Motion describes the effect.
You'll have a harder time getting the slide to move, and once you get it moving, it will tend to want to stay moving. Considering the last 1/4 inch of KSG slide travel... the added momentum of the slide (within reason) might not be a bad thing. Your arm is only so strong, so at some point additional mass will be detrimental to momentum and inhibit the acceleration of the slide.
If you look some firearms designs you see that careful control of mass has been an interesting study. Take, for example, the Colt double action revolver. It has a massive cylinder release that must be pulled rearward to unlock the cylinder. That mass contributes to the "lockup" of the mechanism under recoil. Put another way, the frame moves backward under recoil, and the release wants to stay put, so things get tighter. Colts have a very weak return spring on the cylinder latches. They don't need a strong spring. Smith & Wesson is different. You push the latch forward to unlock. The part is less massive than Colts, and the spring forces are greater - since it must remain locked during recoil. Ruger double actions have a pivoting release, and you can stick one those huge bullseye shaped Power Custom releases on a Ruger and it doesn't effect the operation of lockup, or spring tensions. The Ruger revolvers with "bar-in-frame" crane locks are also using a little momentum to to keep that bar in place in the frame, making things tighter under recoil.
Considering mass and the action of the slide is really an outstanding question, thanks for asking it!
