One of the guys I ride with switches between a lightweight CF bike that weighs around 19 lbs or so (with pedals, bottles, etc.) and an older aluminum cross bike with 28mm tires that probably weighs 24 lbs or so (rough guess but it's probably heavier). On the CF bike, he is usually so far ahead of me by the 20 mile mark that he looks like a speck on the horizon. With the heavy cross bike, he is a slightly larger speck on the horizon, not quite as far away.

The bikes are both heavier than my bike and my friend is older than me. A heavier bike doesn't seem to slow him down much, but I'm thinking about secretly letting some air out of his tires to see if that helps. Will report results.