The latest battle over high-speed trading is being fought out in one of the nation’s federal appellate courts. The subject this time is a new order proposed by Investors Exchange LLC (“IEX”), the Discretionary Limit, or D-Limit, order. IEX’s stated mission is to protect investors, institutional and individual investors alike, from latency arbitrage, a strategy practiced by high-speed trading firms. First, some background. In 2016, the SEC approved IEX’s exchange, along with r