OK, so I feel the need to weigh in. I have two road bikes that I ride regularly. One is a ~15 year old, sub-20 pound pro-level (state of the art in its day) race bike; the other is an I-don't-know-how-old (30-40 years probably), 30-ish-pound full 531db race bike, 650b conversion with VO fenders and dynamo lighting. I average about 2mph faster on the light bike, but I doubt that the bike is inherently faster. More likely I ride faster on it because it feels lighter and livelier. If I had to choose one, it'd be the 650b bike because it's more versatile and I don't always ride on good roads in nice weather. But I'm glad I haven't had to choose. I love them both.

SP
OC, OR