The economy is booming, with GDP growth at 3.5 percent. But worker productivity — the amount the economy produces per-hour-worked — not so much. That’s just 1.3 percent. Worker productivity growth has  been below 2 percent for a decade, well below the historical average since World War II. There’s no clear cut answer as to why the U.S. is so stubbornly slow when it comes to productivity. But, it matters.