The issue with slow execution is that you don't run the test suite often enough.
I like running my tests almost as often as I compile my code. If they run fast enough, it creates the most addictive feedback loop for TDD.
Besides, we should be measuring the time to write tests against the time it takes to manually test. If you don't execute tests every time you make a change, you're not really sure if your code works. And manual testing just means we have no idea whether or not our code works.
