For all the nail-biting about the economy over the past year, investors had it easy: a 5% yield on short-term U.S. debt, one of the safest investments in the world. Now, the times they are a-changin’—again.

Markets on Wall Street were positioned to open at record highs Thursday after the Federal Reserve issued a bigger-than-usual interest rate cut intended to fortify a cooling labor market and prevent a recession.