It is more than likely wave gear vs slalom gear. You're on a mountain bike and they are on racers. Simple as that.

Personally I sail FreeStyleWave gear that is somewhere in-between. Easier to sail and rig and jump and gybe and stuff. I
might get some GPS slalom gear once I've nailed some forwards (be forewarned).
The trick to going fast, me thinks, is to be light.

Once you're up and planing nicely, 3rd gear, you want to start being
slippery and
smooth.
1. Don't oversheet the rig; let the sail get
pulled along rather than pushed. You'll feel it. The rig will actually rake back by itself. You can squeeeeze a little for bursts of speed where you want.
2. You also want the board to glide over any chop, as flat as possible, without crashing through any waves. You'll feel them slow you down. This is where your weight in the harness comes into play. You can kinda float over the rough bits.
Note that slalom gear includes large, upright fins that they can really push hard against for extra traction. It's a significant advantage on any angle to the wind.