If your Python web app is slow, because 80% of the time you're waiting on the database, well, it doesn't really matter how fast your Python code is. The things where PyPy is quite bad at is if you spend most of the time in C extensions,. Then either not helping or actually preventing you from doing so. And then again, on PyPy, it's like over 10 times faster. So those are the very good examples. But as I said, you should really measure yourself and figure this out.