On hedge fund titan Kenneth Griffin’s 46th birthday last October, his firm paid former US president Bill Clinton $250,000 to speak to investors and employees at the New York restaurant Daniel.

As guests prepared to dig into their short ribs and tenderloin, Griffin's thoughts were elsewhere. Stocks were sliding, and the billionaire founder of Citadel saw a chance to pounce. He huddled with Ian Sandler, chief operating officer for global equities.