To me, the issue around bicycle weight is not about "performance" per se but actual portability. There are circumstances in which I regularly need to carry my bike up and down flights of stairs--to get it onto a subway/train platform, or to access the bike path near me, or get it up the steps fronting my apartment building. I also have to lift my bike when I put it in a car trunk. So in these circumstances, yes, a lighter-weight bike can make a lot of difference.