Billionaire investor Warren Buffett told CNBC on Monday that Berkshire Hathaway investment managers Ted Weschler and Todd Combs have trailed the by a "tiny bit" since each joined the company.

However, the Berkshire chairman and CEO points out that they've done better in the market than he has over the same time period. He said it's a tough time to beat the S&P 500.

"Overall, they are a tiny bit behind the S&P, each, by almost the same margin," Buffett told Becky Quick on "Squawk Box" from Omaha, Nebraska — where Berkshire is located. "They've done better than I have."

Weschler and Combs, both former headge fund managers, now manage about $13 billion each.

Combs, in his late 40s, came to Berkshire in 2010 after a three-year search for someone to help manage the company's massive portfolio of stocks. Weschler, in his late 50s, joined Berkshire two years later, after paying a combined $5.3 million to win the 2011 and 2010 auctions for an annual charity lunch with Buffett.

"The first few years, ... they got well ahead of the index and they got paid," Buffett said, noting their compensation is dependent on performance. "It came in thirds. So it could be clawed back, two-thirds of it, if they missed the second year and so on."

"Both of them have done an incredible amount of work in terms of acquisitions; Todd, in particular, on our medical venture," said Buffett, referring to the Berkshire-Amazon-J.P. Morgan venture aimed at figuring out how to reduce health-care costs for their employees and how that may possibly help cut health-care costs in the United States.

Seen as a must-read for investors, Buffett covered many topics in his weekend investor letter, including Berkshire's $112 billion cash pile and what he wants to do with it, his first stock purchase, the dangers of too much debt, and what he sees as the biggest risks.

Berkshire's portfolio of stocks, including top holdings American Express, Apple and Bank of America, was valued at nearly $173 billion at the end of 2018.

Monday's interview with Buffett, 88, was conducted at Berkshire-owned Nebraska Furniture Mart.