Here’s one thing to like about Donald J. Trump’s speech at the Waldorf Astoria Hotel on Thursday: He cast attention on what really is one of the defining economic challenges of our era, slow growth.

Mr. Trump would like to “establish a national goal of reaching 4 percent economic growth,” as he told the Economic Club of New York. He says s his economic plan will generate 3.5 percent growth and 25 million new jobs over the next decade.

Here’s what he gets right. Growth in the United States (and the entire advanced world) has been well below its post-World War II norms since the turn of the century. From 1947 to 2000, the United States economy expanded at an average of 3.6 percent a year. Since then it has been 1.8 percent.

It’s probably true as a society that we’ve become too accepting of sluggish growth, even if everyone has different theories of how to fix it. The problem is that when you dive into the math around jobs and G.D.P., it becomes clear how unlikely the country is to meet Mr. Trump’s goals. And that’s true even if you believe his agenda would be good for the economy.