High-frequency trading has been good to billionaire Ken Griffin.

His Citadel LLC returned more than 300 percent in a fund started as a high-frequency strategy in late 2007, according to two people familiar with the Chicago-based money manager. The $830 million pool, which added other strategies in recent years, beat the 44 percent gain of the U.S. stock market in the six years through 2013 as well as Griffin’s two main hedge funds, which together have $8.8 billion in assets and rose 45 percent in the period.

The returns of Citadel’s Tactical Trading fund give a glimpse of the fortunes made in high-frequency trading -- the rapid buying and selling of securities that relies on ultrafast computers to exploit market inefficiencies -- following the financial crisis, and before profits shrank with increasing competition. The practice is facing unprecedented scrutiny following Michael Lewis’s latest book, “Flash Boys,” which argues that it has helped rig the U.S. stock market.

“High-frequency trading hit its high in 2008 through 2010,” said Larry Tabb, founder of market-research firm Tabb Group LLC. “Then the trades started getting crowded in U.S. equity markets and it got harder to generate revenues.”

Citadel’s Tactical Trading fund jumped about 31 percent in 2008, when the S&P 500 Index of U.S. equities slumped 37 percent and hedge funds broadly lost 19 percent. The fund has returned an annualized 26 percent since the beginning of 2008, with just five losing months, said the people, who asked not to be identified because the fund is private. The worst drop was a loss of 1 percent.

Return Profile

Mike Geller, a spokesman for Citadel at Edelman, declined to comment on the fund performance.

The return profile is very different from Citadel’s Kensington and Wellington funds, which plummeted 55 percent in 2008, a $9 billion loss that the firm didn’t recoup until 2012. The funds, which invest in everything from corporate debt to global stocks, lost much of the money on credit-market wagers in 2008. The funds returned 62 percent the following year as fixed- income markets rebounded.

Griffin, 45, has since seen improvement in his main hedge funds, which have returned an annualized 26 percent since their 2008 losses. His plan to build an investment bank following the financial crisis failed less than three years later amid departures of top executives.

Citadel Securities, its business of filling buy and sell orders for clients, pays retail brokers like TD Ameritrade Holding Corp. hundreds of millions of dollars to send orders its way, Lewis reported in his book, which also describes the unit as using high-frequency trading. Citadel Securities says it executes about 14 percent of U.S. consolidated volume in equities and 20 percent in U.S.-listed equity options volume.