I concur that a given steel frame should feel pretty much the same through its life, until the tubing, the lugs, or the brazing actually begin to fail. A long-wheelbased, slack-angled older frame, engineered expressly for riding long distances on cobblestone roads in Europe, will always feel softer than a contemporary racing frame, particularly one designed for criterium or time trial use. I am only marginally faster on my 26-year-old Bianchi than on my 48-year-old Capo, and the frame makes a difference only on climbs and sprints. For flat cruising, tire rolling resistance and my own aerodynamics dominate, and it really doesn't matter which frame I am on, other than the fact that the Capo is more comfortable and forgiving on the bumps.