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.

The Federal Reserve's closely followed overnight reverse-repo facility already has begun to reflect Wednesday's big interest-rate cut. Wall Street firms rushed to park extra cash overnight at the facility in the wake of the pandemic,