If a running shoe made you 25 percent faster, would it be fair to wear it in a race? What about 10 percent? Or 2 percent? The Nike Zoom Vaporfly 4% — a bouncy, expensive shoe released to the public one year ago — raises these questions like no shoe in recent distance running history. Nike says the shoes are about 4 percent better than some of its best racing shoes, as measured by how much energy r