Just another thing to consider is shifting efficiency. I know that when I ride my brifter shifter bike I tend to shift gears more often than on the non-briftered bikes. Just an efficiency convenience thing, flick the fingers to shift up or down versus letting loose with one hand to shift. Side by side riding probably no real speed difference, but keeping my cadence more fluid and in the happy zone probably adds up to some time savings over a given distance.

But with that said I only have one briftered bike, all the rest have downtube or stem or thumb or grip shifters.
Tires, wheels, gearing, geometry, fit, riding position, shifter format, even the bike's color or paint scheme can have some functional or emotional effect on a bike's speed and efficiency.