Software-Defined Networking (SDN) is a brand-new networking technology which promises to provide flexible architecture and power features. However, performance is always a major concern in deploying SDN. In this paper, we demonstrate how a naive SDN network may suffer from long delay for the first packet in a session, and we propose a "Look Ahead Mechanism" to shorten the delay time. In addition to simulation, laboratory prototypes were implemented to verify the mechanism. The experimental result shows that the delay time can be reduced about 50%, which significantly improves the response time in an SDN-based network.