I’ve been thinking about this topic for a long time. Looking in Veloviewer, my fastest bikes for average speed per ride have been my singlespeeds. There’s nothing special about them, I set them up as road single speed and just rode them like normal. Average ride length, time and elevation are all pretty consistent with my regular geared road bikes. Similar weight too although my State Warhawk was heavier than the others by about 5-7 pounds. SS gearing was 72 or 75 gear inches, 700cx25 tires and fenders.
Anyway, my geared bikes generally averaged 16.2 miles per hour, single speeds averaged 16.9 miles per hour. This is pretty crazy, I think the main thing is that single speeds force a total rest on most mild to steep downhills. It’s easy to spin out a 75 inch gear and be forced to coast to rest. This rest then allows for faster speeds on the flats and climbs. I think it’s the same for mtb, you don’t get stuck in that feedback loop of pedaling all the time where you go too fast on the flats and easy corners but then scrub all this wasted speed and wattage because you came in too hot. Better to just flow for most things and have a lower total entry speed but a higher overall speed.
Conversely, on the climbs the lower cadence allows your heart rate to drop and aerobic recovery to occur. So then when you get to the flats you can blast away pretty well rested.
It’s like how air resistance is logarithmic so you end up spending more watts per mph to try to go from 30 to 31 mph on a down hill in 50×11 or whatever than you would if you were trying to go from 16 to 17 mph on the flats.