Assert that one run is significantly faster than one other

I need to make a functionnal test that assert that one run is significantly faster than one other.

Here is the code I have written so far:

def test_run5(self):
    cmd_line = ["python", self.__right_def_file_only_files]
    start = time.clock()
    with self.assertRaises(SystemExit):
        ClassName().run(cmd_line)
    end = time.clock()
    runtime1 = end - start

    start = time.clock()
    with self.assertRaises(SystemExit):
        ClassName().run(cmd_line)
    end = time.clock()
    runtime2 = end - start

    self.assertTrue(runtime2 < runtime1 * 1.4)

It works but I don't like this way because the 1.4 factor has been chosen experimentaly with my specific example of execution.

How would you test that the second execution is always faster than the first?