The most significant factors affecting vessel energy consumption are vessel average speed and speed profile. For container vessels, optimizing speed profile can be a problematic issue because of changing harbor time slots which requires dynamical changes to speed profile. In view of optimal speed profile, it is crucial to have knowledge of time slot changes as early as possible. Early knowledge of delays allows better speed profile optimization, since effect grows significantly towards the end of the leg. We compare the effects of early arrival in different ports, and suggest an optimization scheme that can be used to optimize speed profile with changing schedule information. The data used is collected using real time data collection system onboard several container vessels.