How do you define performance requirements in SRS? Unfortunately some of our clients have asked us to fix this point beforehand.
For example, a requirement like "The system has to support 100 concurrent users" is very far from the real world because in the real world we operate with other things like browser requests and customer sessions; we have concurrent httpd processes which process these requests and handle these sessions; we have incoming user requests and different interaction rate; and it sounds blurry what is "has to support" in this requirement, etc.
It might be someone of you has got the relevant experience. We made performance test for our current clients but their systems were different from the projected one and it is difficult to project numbers before the development/testing stage.
Rauf