By Telis Demos

Capital One Financial Corp. reported another big quarterly loss Tuesday, as it set aside $2.7 billion for potential credit defaults. In light of the Federal Reserve's new bank dividend policy, the lender now expects to slash a planned 40 cent quarterly dividend to 10 cents. There's potential opportunity for investors in beaten-up card lenders. But it may not pay off for a while.

With Capital One's big reserves socked away and a dividend cut priced into the stock, it is tempting to believe that the worst might be over, especially if more stimulus is on the way to help borrowers keep making payments. Like Ally Financial, Capital One's still-strong credit performance measures and mostly positive experience with forbearance programs do suggest that on present trends, credit losses may not approach the dire scenarios banks are reserved against.

To keep paying a dividend, should the Fed retain the policy through the end of 2020, Capital One would need to earn over $1.1 billion in the third quarter, the bank figures. That is certainly possible, provided that reserving slows significantly--it earned at least that much in every quarter of 2018 and 2019.

But reserves aren't the total earnings story. Capital One also reported a sharp drop in net interest margin, a full percentage point from the first quarter to the second quarter. One driver of that is lower interest rates. Another is that the same stimulus helping consumers avoid default is also leading some to save more and pay down credit-card debt. The bank described this as "the flip side of good credit" and a "hidden factor" in slow growth.

That means cardholders are borrowing less, and that Capital One isn't earning the big yields associated with card loans. The bank's average card loans fell 11% from the first quarter to the second, offset by an uptick in auto and commercial loans and by a surge in cash from deposits. The bank says deploying that cash could help soften the net-interest-margin trend in future quarters.

Still, this dynamic puts investors in a tough spot. If government stimulus is extended, Capital One and other card lenders may not add much more to allowances, helping future earnings. But unless the economy is also concurrently picking up to drive spending and not saving--a big question mark right now--stimulus won't fully relieve the pressure.

As a committed consumer lender, Capital One could nab market share, if its big-bank competitors are forced to be more cautious about balance sheet growth. But that is a longer-term story, and investors can expect a bumpy ride.

Write to Telis Demos at telis.demos@wsj.com