Passes a property based test but now your game doesn't actually run because it's much too slow of an operation on hardware at that time.
You can also test execution time too, but that's finicky and doesn't help explain how to fix it if you break that test (if there's no accompanying documentation).
Performance can be tested in a unit test. You just need to measure the time needed to compute the function on a given set of numbers, then measure the time needed to compute 1.0f / sqrt(x) on the same set of numbers. The test succeed if your function is 10x faster. In future, the test may fail because sqrt has improved and this trick is no more needed.
You can also test execution time too, but that's finicky and doesn't help explain how to fix it if you break that test (if there's no accompanying documentation).