America's growing thrift is proving to be a drag for credit-card lenders. Since the height of the financial crisis, a big worry for lenders was that cardholders were falling behind on payments or defaulting on them altogether. As those trends stabilize, a new problem is emerging for card issuers, which profit by collecting interest on unpaid balances: U.S. borrowers, emerging from the recession, are paying off their credit-card debt more quickly.