The U.S. presidential candidates have made the usual pile of promises, none more predictable than their pledge to make the U.S. economy grow faster. With the economy struggling to expand at 2% a year, they would have us believe that 3%, 4% or even 5% growth is within reach.

But of all the promises uttered by Donald Trump and Hillary Clinton over the course of this disheartening campaign, none will be tougher to keep. Whoever sits in the Oval Office next year will swiftly find that faster productivity growth—the key to faster...