The physics behind the cycling of a locked breach handgun are quite simple. You have two major forces resisting the motion caused by the force on the breechface (caused by gas pressure):
1. The force caused by the spring. This force is directly proportional to the amount it is compressed. At the beginning of the cycle, this force is at it's lowest because the spring is at its least compression. This makes the second component important.
2. The force caused by the slide mass's resistance to motion. This is the old F=MA (Force equals Mass times Acceleration) equation. The lower the mass of the slide, the less force it takes to accelerate it. Or, more appropriately in this situation, at the same force (chamber pressure), slide acceleration (and velocity)will be much higher if the mass is lower.
Increasing the spring weight probably won't solve all of the problems. The XD's locked breach mechanism uses a cam on the barrel to unlock the barrel. The angle of this cam is designed to keep the action shut until a certain point in time AFTER the bullet has actually left the barrel and chamber pressures have dropped to residual levels. If the action unlocks while the bullet is still in the barrel and chamber pressures are still high, you will get a ruptured case head, and all kinds of problems.
In other words, if you dramatically change the mass of the slide, the locking mechanism (cam, etc.) will probably have to be redesigned.
I'm not sure if you are seriously considering this project, but if I were you, I wouldn't even attempt it.