I'm not saying it is that easy, but from a mechanical grip scenario, the more surface grip there is, the less grip you need in the tires. The less grip you need in the tire, the lower the static and kinetic friction has to be on the kart side of the relationship. If the kart needs less kinetic grip, then the kart can roll freer across the surface to maintain the same grip level.
The harder tire, either by air or by duro, reduces rolling resistance. The less rolling resistance the higher the percentage of horsepower to the track. As free horsepower becomes available, you go faster. This continues up to where you start to lose said grip, then the free horsepower is translated into wheel spin, which we all know means less speed.
These things free up the speed in the kart.
NOW WITH THAT SAID, if its on the kart side where grip increases, like by putting my 240lbs in the seat, then yes, it may only hurt the cause.