I would that taking the time to test ride is very important. try to have it adjusted to fit you...stem height, stem length and seat height. adjusting the seat is easy, raising a stem could be tought, and some shops won't want to put a shorter stem on for your test.

some bikes are faster than others...the first thing to look at are the tires. there is a big difference between racing tires and "sport" tires, they may not look too different, at first glance, but a racing tire pumped to 160 psi will feel faster.

wheels make a big difference also. record and dura ace hubs feel faster than entry level stuff. in fact the quality of all the bearings in the drivetrain can make a ride feel faster than entry level stuff.