Since the late 2000s, however, China seems to have lost a lot of its dynamism. The International Monetary Fund estimates that total factor productivity – a measure of the efficiency with which resources are used – has grown only half as fast since 2008 as it did in the decade before. You should take such estimates with large handfuls of salt, but there has been a clear slowdown in the rate of technological progress. And China no longer has the demography to support torrid growth: Its working-age population topped out around 2015 and has been declining since. Many analysts attribute...