Originally Posted by badger1

I have, and do. To respond to your question: some, but much less than many would have you believe. I have two reasons for saying this.

1. Personal experience. I do all my road cycling on a flat-bar road bike (pictured below). I don't 'race' or do group rides; I do all my riding solo. That said, I've ridden two imperial centuries on this bike; didn't have any problems. I routinely do 50+ mile rides at least once a week during the season. I've never noticed any particular disparity between my speeds and those of other cyclists of a similar age/apparent fitness (I'm 66; pretty fit aerobically) who are on full-on drop-bar road bikes.

2. Data. I've trotted this out before, but it bears repeating. I know of only one reported genuine, disinterested attempt to compare 'drop bar' with 'flat bar' road bikes. This was a test undertaken by Brit mag Cycling+ a few years ago: the testers rode two endurance-geometry drop-bar bikes and two flat-bar road bikes -- equivalent quality, weight, tires etc. -- over a period of time over a known course approximating a 50-mile sportive. Lots of hills, lots of descents, some flats, etc. They tried to control for actual effort through hr monitors.

Result: there was a disparity, but much less than they expected to find. They found they were about 5-6 minutes quicker around the course on average on the drop-bars for the same (roughly) effort. What was interesting: they found they were quicker up the climbs on flat-bars (they attributed this to more open breathing), but that this was more than offset by their being quicker on the descents on drop-bars when in the drops: lower centre of gravity (cornering speed) + more aero. They did point out that they used the drops extensively while descending, and that in their opinion that advantage would be lost for folks who don't.

So, not a full-blown 'scientific' test but as I say it's the only attempt to make a rational, empirically-based comparison, fwiw.