I stumbled across a nice article at queue.acm.org titled Thinking Clearly about Performance.
If you’re interested in some theory behind performance optimisation, this is a must read – although there’s a good chance you know some parts of it, if you’re interested in the theory behind .. you get it.
The article talks about response time vs throughput, percentile specs, histograms of method calls with improvement cost and queueing theories (M/M/m).