This might seem like a stupid question, and I posted it in here rather than Road Cycling because being a lot heavier than most of the folks in there I'm not interested in replies that only apply to people who look like beanpoles.
My normal bike is a Specialised Tricross. I run 700x32 Schwalbe Marathon Plus tyres on it, leave a rear rack permanently mounted (for convenience, it's a bit of a faff taking it off and on). For the most part I can cruise reasonably comfortably at speeds between 15-20mph on the flats, usually closer to the low end but on a good day can cruise in the high teens for a while. The gearing is 50-39-30 at the front and a 9-speed 11-32 at the back.
The owner of my LBS lent me a bike built for speed. It weighs a good bit less than the Tricross, has 700x23 tyres, and I think the gearing is 53-39 at the front and it's got a tight 9-speed cassette at the back (I haven't counted but I'd hazard a guess it's 11-25 or similar). On a test ride today cruising at 15mph felt like it was no effort at all, and holding speeds in the low 20s didn't feel overly difficult. Local hills that are quite hard with my 30-32 gear on the Tricross were comparably difficult in terms of perceived effort in the 39-25 (give or take) on the lightweight bike.
So here's the bit that's confusing me. Obviously the speedy bike is lighter than the Tricross but by the time you put 250lb of me on the top the difference in bike+rider weight is going to be small. The speedy bike has something like 20 bladed spokes rather than 32 round spokes but it's hard to see that making such a difference. I can see going from 700x32 to 700x23 tyres would make a difference, as would tyres lighter than the Marathon Plus I normally run.
I've got two bikes already and would seriously struggle to justify a third one to my wife, who is very gracious about bikes living in the kitchen. If I can get a good performance boost by putting lighter wheels and thinner tyres on the Tricross that's certainly something I'd look at doing, even if I kept a spare wheel with a fatter tyre (my existing 700x32 in all likelihood) for the times I want to take it on more rugged terrain or load up the panniers.
Given the state of my local roads I'd want tyres tougher than the Continental 4-season tyres that were on this particular bike. I ran over something that went straight through the sidewall, trashing the tyre (almost brand new) and the inner tube. So whatever the performance benefits of going uber-lightweight I'd rather lose some performance and not have to carry a spare tyre around with me.