Lower Top Tubes on Road Bikes My girlfriend wants to buy a used road bike as well.  She is pretty small 5'2'' with shorter legs so it has been tricky finding anything that would fit her.  

She suggested that we should look at something with a lower top tube.  Ie something like this: 



(Not sure why but the images don't appear to be working.  Here is a link: http://massachusetts.inetgiant.com/Boston/AdDetails/COUGAR-10-speed-women-s-road-bike-NH-MA-border/8075274)

I don't know why but this does not seem like a serious bike to me.  We want to do some faster riding but I don't know how to justify why this isn't a "fast bike".  (Or is it a fast bike?)

I know some benefits of having a regular top tube are: 


the bike has less flex and will lose less power on each pedal stroke.  (Is this significant?)
The geometry is right for the person riding the bike.  If you buy a bike with a lower down tube you are not really solving the problem.  With a solution like this you will not be able to get your handle bars at the right height (ie: at the same height or lower than your seat post).  


Question: Will a bike with a regular top tube be faster than a bike with a lower slanted top tube?  Why?
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.