It depends what kind of old 10-speed you're talking about.

A high quality road bike made of race-quality double-butted chromoly tubing and with decent mid-range to high-end components wasn't exactly the same thing as a 30-35 lb steel department store 10-speed. Once you get down to under 25 lbs, you've got a pretty decent road bike that you can have as much fun riding as anything you can buy today (and especially so if it's from at least the 1980's). Fitness and a good efficient riding position will still make you faster than most people are on their lighter carbon bikes unless they are as fit as you. But most people aren't that fit, no matter how much money they spent on their bike.