Lower Top Tubes on Road Bikes
Frame flex. The step-through frame means that the seat tube has a bending force applied to the middle of it, so it has to be stronger and heavier than the same tube in a diamond frame. Usually the down tube is also beefed up. Even so, there will be more flex both laterally and longitudinally/torsionally. I've also not seen a decent lightweight frame in that geometry. If you want light in a step-through design perhaps look at a mixte frame instead.

You can get some seriously tiny road bikes these days (google). The real choice (IMO) is between 650c and 700c wheels. 650c wheels look better, make for a lighter bike and generally have fewer compromises. They are more of a scaled down full size bike. 700c wheels mean the bike looks stretched a little with the rider low down between the wheels. The advantage is that tyres are easier to find and the manufacturer already has jigs etc to build bikes around 700c wheels. Tyres are only slightly easier, these days Schwalbe and others make a decent range of 650c tyres (they're not just a track &amp; time trial size) and shopping online means you can definitely buy them (your LBS should be able to order them in too).

To find one of those second hand I suggest checking out the second hand ads at your local racing clubs. Finding one in the giant mess of second hand bike ads has proved tricky for us in the past, mostly due to the overwhelming number of cheap crappy childrens bikes that pollute the search results. However if you want to sell one just approach short adults... you'll sell it very easily.