Here is a link to another document I wrote regarding Response Time --> http://ibm.biz/drt-basics
Abstract: This article deals with response time: its definition, how to identify and analyze the fundamental factors that it depends on, and how to perform "what if" or prediction analysis based on a very simple model. Bear in mind that we're going to discuss here applies to generic systems, not only information ones. So we won't be stuck on these, but instead we will also go to hairdresser's and will have a (performance) look on our own department team.
Hope you like it!
Jorge L. Navarro