It matters. For me it's not so much the average speed as the perceived effort on climbs. I can loafcycle all day on a lighter bike, barely pushing the aero limits around 16 mph, but be less tired because I'm hauling less weight up every climb, and expending less effort on short sprints.

My 1993 Trek 5900 has been out of service most of this year waiting for parts for a long overdue overhaul. So I've mostly been riding my '89 Ironman steel bike. The 5 lbs difference is really noticeable on our roller coaster roads with lots of short, steep climbs. I'm slower and more tired after 20-30 mile workout rides, especially in summer heat.

If our roads were flat, or I only rode the MUP, it wouldn't matter much. The limit is aerodynamics above 16 mph, not so much the weight. Once we get rolling on flat terrain the bike weight matters less.

Another factor: While I could handle the standard 52/42 chainring on the older carbon frame Trek, I had to switch the Ironman to 52/39 and am considering switching to 50/38. That's with the same 13-28 freewheel and cassette rear wheels. That's the difference another 5 lbs or so makes.

And with my 30+ lb Univega, I need the 30T small chainring often with the 28T rear cog, on the same climbs I do with the 39T chainring and 24 or 25 rear cog on the Ironman. That extra 5 lbs on the Univega feels much more significant than the 5 lb difference between the Trek and Ironman.

Reminds me, I have a very lightweight Diamondback Podium carbon frame to build up. Just slowly gathering the appropriate components to keep it light. Should make a big difference.