Talking about Web Server Performance is a serious matter and for sure is not an easy one to perform. We can safely say that it is one of the few things that make the difference when we are involved in a professional site project development.
One of the main questions that the client ask is how many simultaneous visitors can the site handle. Well, we are all know that this is a very general question and can be affected by many factors. To state our client's question in a more technical view, let say this:
We want to check our web server responsiveness when many simultaneous requests hit on it.