My experience is that my 18.5 lb Carbon Fiber bike climbs about 5-10% faster than my ~ 21.5 lb double-butted steel bike, configured similarly, with me as the rider, depending upon the gradient I'm climbing. For anything other than climbing, the performance difference is negligible. That 5-10% difference in climbing performance translates to maybe a 2% speed difference overall on the amalgam of all the terrain that I ride.