Vendors are rarely believable when data sheets state the performance of IPS systems. That’s partly the fact that IPS performance is variable according to types of inspection rules that are configured and the amount of traffic hitting those rules, in use. In short, IPS performance is determined by the complexity of the rules you choose to configure.
Cisco has provided some insight into these criteria so that you can plan
This white paper illustrates how different IPS performance metrics can be used to help customers determine the appropriate sizing for their IPS deployments. Customers can use this information to help interpret the different performance numbers that are presented in the Cisco® IPS 4500 and 4300 Series data sheets. The paper also provides detailed performance results for various testing methodologies.
I would recommend reading this to learn more about how hardware and software systems combine to determine the actual performance of systems that use generic Intel processors and I/O in addition to some customer silicon for specific tasks.