Network intrusion detection systems rely on a signature-based detection engine. When under attack or during heavy traffic, the detection engines need to make fast decision whether a packet or a sequence of packets is normal or malicious. However, if packets have a heavy payload or the system has a great deal of attack patterns, the high cost of payload inspection severely diminishes the detection performance. Therefore, it would be better to avoid unnecessary payload scans by checking the protocol fields in the packet header first, before executing their heavy operations of payload inspection. Furthermore, when payload inspection is necessary, it is better to compare attack patterns as few as possible. In this paper, we propose a method which reduces payload scans by an integration of processing protocol fields and classifying payload signatures. While performance improvements are dependent on a given networking environment, the experimental results with the DARPA data set show that the proposed method outperforms the latest Snort over 6.5% for web traffic.