By the way, I've always been disappointed by the difference between my effortful speeds on a "fast light bike with fast tyres" and the other sort. On the whole, less than 1mph. Is it me??
Mathematically speaking, bicycle speed is largely a function of power output vs wind resistance (mostly of your body) and on particularly hilly rides power vs weight (again, mostly of your body). While the style of bike can definitely influence those things, for the same rider on two vaguely similar bikes they're going to work out pretty close.
Rolling resistance, bicycle weight, gearing, even the weight of a bit of luggage has a much smaller influence than many
road club bores people who've watched too much GCN people assume.