My experience in London, is that in start/stop traffic light hopping there's little difference between a light road bike and a folder – other than the light road bike (with muscle-bound hero) gets away faster and might beat more sets of lights. What the light road bike makes up over the heavier folder, it
could lose at the next red light. The smaller wheel folder though, benefits greatly from more nimble handling in densely populated cities. And it folds into luggage.
Out in the sticks my 9 Kg road bike covers the same 10 mile round trip to the next village in about half the time it takes on my 20” folder and I'm no athlete. Larger wheels mean I'm not slowed so much by rough surfaces, and 21 gears instead of 7 mean I'm always spinning, not mashing. My 15 Kg MTB is slower than my road bike over the same trip, but faster than my 13 Kg 20” folder - it's rough-surface tolerant, and my folder isn't.
The stated aerodynamic benefits of small wheels (and frames) are much overstated for urban cycling, which often needs fat, heavy (in comparision) glass-proof tyres. By the time the bike gets to 20 mph, about 80% of the energy lost is to the cyclist's and bike's frame drag. In London (but not perhaps other cities) , it's hard to maintain 20mph for any distance, thus reducing the already small benefit.
More powerful riders leave less powerful riders standing, too. Very noticeable in towns, regardless of what people ride. Fast acceleration makes up for a lot in urban areas.You want fast, you need muscles, aerodynamics, planned diet, fast clothing, light bikes, un-lardy healthy riders with muscles in the right places, and road-tolerant wheels and frames. Pick any two.
I'm trying for health and light weight. I don't always succeed. I cycle most days on my road bike and I'm still overtaken by guys on Bromptons.
The bible of what goes faster than what is 'High-Tech Cycling'. It's
here. The book makes the point that fitting aerodynamic disk wheels is of no benefit for hill climbing, and light weight is of little benefit on hill descents. The best accessories for speed are health and fitness.