Specifying Performance - Unweary
http://unweary.com/2008/11/specifying-performance.html

Great essay describing the perception of software performance and the various time ranges that matter. Good content and great source of references to other source material on the topic.

Designing and Engineering Time

When you need to choose which part of your application to focus on speeding up, understanding where and why users will perceive performance problems is key. You can't and shouldn't optimize everything. Remember, perception is reality. No mater what your metrics say, if the user thinks your application is slow, it is.

"Failing to specify reasonable performance requirements makes it very difficult to verify that your software is actually meeting your users' performance expectations." … with a good overview of Seow's timings.

very useful thoughts on setting performance expectations.