Raj Mahajan achieved a rare when he rejoined Goldman Sachs last year with the coveted title of partner, the Wall Street bank's highest rank. Then he got to work on fixing the pipes.

That plumbing has to do with the technology Goldman uses to route stock orders to exchanges and private trading pools. The bank has long been one of the top two stock brokerages in terms of revenue and customer rankings. But in recent years Goldman's status slipped in electronic trading because its technology did not keep up with client demands for ever-faster trades.