I used to commute from work to home daily, many days with a work pal. We each had too many bikes, from 30 pound tank commuters to 17-pound (and less) Uber race bikes. In terms of legs and lungs, we were evenly matched.

When either of us was on our #1 light bikes, there was no comparison. Little difference on the flats, but on the first hill on the ride, you could immediately tell who was on the light bike. The poor rider on the heavy bike got shelled off of the back like he was going backwards. Every time. Many times we test-swapped bikes with each other, as we were the same fit: same result.

10 pounds on a 200 pound bike/rider system doesn't seem like much, but a light bike is a revelation in terms of climbing. It produces far more of a difference than makes sense - purely based on the numbers. The biggest difference: light wheels. A 1,200 gram carbon tubular wheelset is a stunning upgrade; you won't believe the transformation in the bike. My 1973 steel Gitane with these wheels feels faster than the 2016 Giant Defy carbon bike I just test rode, that features heavier clincher wheels and disks.

BTW: Test riding a 15 pound bike (with 1,200g tubulars) is a profound religious experience that will spoil you for the rest of your life.