If a company has too much debt and too little income, it's going to struggle to pay its bills, regardless of when its bonds come due.

This is a lesson that investors are learning as distressed U.S. bonds suffer their worst performance since 2008. The notes have plunged 7.5 percent so far this year and 3.2 percent this month alone, with some of the biggest losers being the debt of Lightstream Resources Ltd., Peabody Energy Corp., and Cliffs Natural Resources Inc., according to Bank of America Merill Lynch index data.

It's a painful wake-up call for investors who've gotten used to high-yield debt being synonymous with big returns in an era of unprecedented Federal Reserve stimulus. Even as companies have pushed back debt maturities and lowered interest expenses, that doesn't mean they can continue to work magic in the face of a slowing world economy and oil prices that have fallen 51 percent since the 2014 peak.