High-speed trader Virtu Financial Inc. is pushing back against critics in Washington who say the stock market is rigged against small investors. Virtu’s business of executing individual investors’ orders is facing scrutiny from lawmakers and regulators following the surge in shares of meme stocks such as AMC Entertainment Holdings Inc. and GameStop Corp. Last week, the new chairman of the Securities and Exchange Commission, Gary Gensler, stated that he asked SEC staff to explore changes to the rules governing how investors’ orders are handled. The review will include a practice known as payment for order flow, in which brokerages send many of their clients’ orders to trading firms in exchange for cash payments. Virtu’s stock sold off sharply after Mr. Gensler’s remarks.Payment for order flow has been around for decades and has come under inspection before. But it received new attention after the fierce volatility in GameStop shares in January. At one congressional hearing in February, Rep. Sean Casten referred to Robinhood Markets Inc.’s practice of sending orders to high-speed traders as “a conduit to feed fish to sharks.”