It's all perception:

When I got my 19 lb. Klein, it felt like a feather under me, compared to the 25 lb. bike I was used to riding. Now that I'm used to the Klein....I can no longer feel the difference between the two bikes. And there never was any performance difference.

They did a study years ago: It takes 12 lbs. to make a 1 MPH difference, all other things being equal. 12 pounds!