Performance
Gauging performance is always difficult because no matter how identical the systems to be tested are, minute changes in their configurations can have a dramatic effect on performance. It is possible to pick a "winner" based on bare-bones installations of both systems on the same hardware, but this doesn't really reflect the true performance capabilities of a system in the field. It's equally possible to configure one Apache and one IIS system to be either over- and under-efficient, again skewing the potential results.
Instead it's better to look at the potential capabilities of each system. IIS incorporates a wide range of features designed to improve performance. The worker process execution model provides significant performance benefits on multiprocessor machines, and the ASP and ISAPI extensions enable applications to be executed directly from within the processes handling the requests. The kernel mode driver, HTTP.sys, is also capable of serving pages directly from the cache (memory or disk) for both static and dynamic components, eliminating the need to send the request to a worker process. IIS automatically caches pages generated from dynamic elements to speed up responses.
Under Apache, the same range of facilities are present. Much information can be cached, and tools like mod_perl and mod_php enable the execution of dynamic and template pages almost as quickly as static pages. Like ASP and ISAPI filters under IIS, the modules effectively place the interpreter directly within the Apache executable, eliminating the need to run an external application and greatly improving performance.
