Such as by using CFFI? Cython's strength, in my opinion, is in the fact that it is its own language resembling both C and Python that compiles to down to C, rather than just an FFI.

> Are your extension modules doing some heavy lifting themselves, or are you using Cython just to handle the glue between the language interface?

I would say the majority of the application logic is written in plain Python, and Cython makes interfacing with it, creating higher level interfaces, and marshaling data far easier than with CFFI or using the C API directly. This is pretty subjective though.