Weight is a factor in this for sure. The heavier rider will accellerate much quicker. It is like a heavy tractor trailer rig going down a steep grade, they will runaway if not driven carefully. As you get better, you can point the board and increase your speed, but let that come naturaly. Competetiveness is fine, but sometimes people can become obsessive with it; just have fun riding and progress.
Actually, leaning back makes you go a little faster. If you are on a gentle slope, try leaning way back to butter your board and get the nose up off the snow; it will actually accelerate. Wax for conditions and make sure you properly scrape and buff the wax. Also a structured base can make a huge difference. Avoid the crouching stance though; it is bad form to be in that "goriila stance" with your knuckles dragging the snow and really throws balance off. If you are worried about wind drag, concentrate on riding properly with your shoulders over the board, not counter rotated to face forward.
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