In my job we have some big projects on Python 2 and Cython gives us access to some of the "new Py3 features": types (real types, not just hints) and async IO (through C primitives). We eventually chose Cython because at least we can get big speedups (depending on the code and how much can be properly cythonized, of course). With Py3 it was a change to the codebase (which we also had with Cython, of course), to get approximately the same performance, maybe slightly less, maybe slightly more.