Investors who have ploughed money into private equity have made an average annual return of 15.3 percent in the 10 years to June 2019, according to Bain & Company. That compares with the 15.5 percent they would have made putting the money in the S&P 500, the report found.

Returns of 15 percent are still "good numbers," said Brenda Rainey, senior director with Bain & Company's global private equity practice, who added that it was not easy to compete with a US stock market that had produced "incredible" returns.