Even as interest rates have risen and borrowing costs for new mortgages have doubled or tripled, homeowners who locked in the enviable rates of the early 2020s have been living blissfully in the past. Moreover, the inflation that prompted rates to rise has bitten chunks out of the real value of their debt.

Alas, the question: Repay debt?

For those with spare cash to hand, a question that seemed remote a few years ago is suddenly a great deal more pressing. Should they repay the debt early? A rule of thumb is if you can invest it at higher returns than the cost of your debt—then, no.

Since the start of 2005, measured in pounds sterling, the MSCI World share index has generated annualised nominal returns of above 10%. Had your mortgage rate stayed the same for the next two decades (though in reality it would have fallen), paying off £1,000 ($1,250) in 2005 would have saved a respectable £1,800 in interest. Investing it in a global share-tracker fund would have made a profit of £6,500, however.

Twenty years ago, for example, the price of the MSCI World index was 22 times as high as its companies’ long-run average earnings. Now that multiple has risen to 30. Put another way, a big share of the gains came from sentiment rather than earnings. Unless investors continue to assign more and more value to each dollar of corporate profit, such a multiple limits the scope for future outsize returns.

Borrowers who were sufficiently shrewd, or lucky, to secure ultra-low rates for decades rather than years will be enjoying their holiday for some time to come. For plenty of others, reality beckons.