When its customers buy and sell stocks and options, Robinhood routes those orders to high-speed traders, which pay the startup brokerage for the right to execute many of those trades. That business, known as payment for order flow, generated about $331m in revenue for Robinhood in the first quarter, according to a securities filing late last week. That is more than triple the $91m Robinhood brought in from payment for order flow in the first quarter of 2020.