When I ride a 1980s steel road bike, we (bike+rider) weigh about 75 kg. On a modern carbon-fiber bike, we weigh about 71 kg. That's a 5% difference, but the latter feels like it has rocket boosters when I get out of the saddle to sprint up a climb, while the former feels like I'm carrying an anchor the whole ride. Borrowing a friend's hill climb fixie machine well under the UCI weight limit gives even more of an exhilaration of speed. But conversely, adding 30lbs of bags to go touring doesn't feel that much more difficult once you're up to speed.