U.S regulators may relax rules that require the fastest possible execution of securities trades, potentially helping upstart trading venue IEX Group’s plans to become a full-fledged stock exchange. IEX, described in author Michael Lewis’ book “Flash Boys: A Wall Street Revolt” earlier this year as a place for investors to place buy and sell orders without worrying that they are being “front-run” by other traders whose order transmission speeds are faster than theirs.