Anthony at UXMovement (emphasis his):
Top aligned labels have proven to be faster and easier to fill out than left or right aligned labels. This is because top aligned labels require half as many visual fixations than left or right aligned labels. Top aligned labels also allow users to move down the form in one visual direction, instead of two visual directions with left and right aligned labels. This makes filling out forms quicker and easier.
Even though he doesn’t provide the proof to back up his claim, this is a generally accepted concept and one that Luke Wroblewski writes more about in his excellent book Web Form Design: Filling in the Blanks.
It’s worth mentioning that speed isn’t always the best thing for your forms though. For a registration form, sure, get through it as quick as possible. But there are times when you might want to make your users slow down and think about the information they’re entering—like a bank’s bill payment form for example.