Interactive Brokers is firing on all cylinders as its clients ramp up their activity in the financial markets.The S&P 500 is a prestigious market index that includes 503 stocks from 11 different economic sectors. It has a very strict entry criteria, which requires companies to be profitable and to maintain a market capitalization of at least $22.7 billion. But even after clearing those hurdles, a special committee still has the final say over which companies make the cut.Interactive Brokers (IBKR 2.87%) ope ...