OpenFlow has been envisioned as a promising approach to next-generation programmable and easy-to-manage networks. However, the inherent heavy switchcontroller communications in OpenFlow may throttle controller responsiveness and, ultimately, network scalability. In this paper, we identify that a key cause of this problem lies in flow setup, and propose a Control-Message Quenching (CMQ) scheme to address it. CMQ requires minimal changes to OpenFlow, imposes no overhead on the central controller which is often the performance bottleneck, is lightweight and simple to implement. We show, via worst-case analysis and numerical results, an upper bound of performance improvement that CMQ can achieve, and evaluate the average performance via experiments using a widely-adopted prototyping system. Our experimental results demonstrate considerable enhancement of controller responsiveness and network scalability by using CMQ, with reduced flow setup latency and elevated network throughput.