Doing TDD You need to know which tests are making whole suite slower. If time used by rake test becomes too long, You may be tempted to skip running tests… Waiting for them to complete becomes suddenly burden on Your way to next task.
So, when You need to optimize Your test suite, test_benchmark is a way to go. Just install Rails plugin and after rake test You will get 10 slowest tests and timing of all test in test.log. Now You know where to look for a good place to start and not to wait too long…