Load Test Often!
Having performance testing results from frequent points in the development timeline can help developers understand the performance impact of various code and system changes. When testing early in the development process on a test rig that is not equivalent to production, the performance numbers are not valuable in their own right, but _changes_ in the performance numbers can extremely valuable. These changes can reveal newly-introduced performance bottlenecks that should be investigated.
For each new development iteration, the previous test (which does not exercise the new functions) should be repeated to determine if the changes had an effect on the performance of previously-tested parts of the system. Then new test scenarios should be added to exercise the new functions and a second test executed to both assess the performance of the new functions under load and provide a baseline for the next test iteration.
In addition to alerting developers to unexpected performance changes, the overall performance trend during the development process can help project managers assess the overall state of progress and determine if more resources should be allocated to analyze and implement performance improvements.
Chris, Chief Engineer