We recently had a web app that went out to site acceptance testing where they found severe performance problems related to request size (massive viewstate ASP.net).
We need to ammend our testing strategy to include performance testing, can anyone give us guidance on best practices please?
This is a very broad case to cover, but here are a few of the highlights of things that we do on a regular basis.
DO NOT just test on your network, get remote testing in. LAN connections are very fast, large pages and large load times can go by un-noticed. Ideally get to a place where it mimics the production location in regards to hardware and proximity/connection to the end user.
Use ANTS Profiler or similar tool to profile for expensive methods, and high memory usage.
Test with multiple users, to simulate load. Depending on the nature of the application also load test, either with multiple physical testers or with testing tools that allow you to simulate and script a load scenario.
Review the code to see if objects are retaining viewstate when they shouldn't need to.
I don't know a hard and fast set of "rules" but I find these are good starting points.
In addition to Mitchel's comments above I would recommend conducting load testing as part of your Continuos Integration (CI) process. Visual Studio Team Suite (Test Edition) contains a good load/stress test tool.