What is your timeframe when you trade intraday only ? 1 min or 5min or second charts ? What is you average trade net profit in percentage per stock ? Just I can get a better imagine for the overall profitability. Do you want to say something about your lookback period, how long it is ? Did you try the same setup for Nasdaq100 Index constituents/stocks too ? When you looked for cointegration did you factor in same sector or same industries stocks too or did you just took all the S&P500 stocks and did not respect any same sector relationship ? Because the overall sharpe is lower when you do not factor in sector or industry relationship in addition to the cointegration measurement.
First. Tick based. Then decision about limit order placed / cancel each second