I agree that performance tests cannot be unit tests but there is no reason we cannot have another set of tests called performance tests. Broadly the tests fall under two categories
a) Unit tests
b) Integration tests
We run integration tests again the real database (instead of in memory) to ensure the sql scripts, the hibernate repositories work as expected
My idea is we can add another set of tests called performance tests which are a part of nightly build which tests for performance of certain functions. This is important to track down the statistics after a code re factoring or to evaluate if changes to one part of application can have unintended consequence on another.
I have come across JunitPerf which might help me to achieve this objective.