Two ways to play this, either fly the CI given on the flight plan and arrive on stand late, then the punctuality stats take a hit, and to be honest I think passengers care more about arriving on time, than the actual departure time, and a late arrival will play havoc with some of the tighter connections. Or to quote TCAS " Protect the Hub" take a couple of extra tonnes of gas, wind up the speed and fly low & fast to try and achieve an on time arrival, and increase the fuel bill.