My lighter bikes go faster because they are also more aerodynamic and have lighter wheels and tyres and when I am climbing hills on my road bike I am only moving 165 pounds up that grade whereas when I am riding my extrabike or some of my commuter / utility bikes I am probably looking at moving as much as 200 pounds up those grades and do that on heavier wheels and tyres.

I am one of those guys who could use a bottle full of lead shot for descents.