At a closely watched hearing before the Financial Services Committee, Robinhood’s Vlad Tenev and Citadel’s Ken Griffin took fire on issues ranging from trading halts provoked by capital shortfalls to whether “free trades” are really free. At times, the chief executives’ long responses were cut off and met with derision. They were both adamant that their businesses have helped small-time investors access markets that were long the domain of Wall Street. “You are doing a great job of wasting my time,” Representative Brad