He Arthur, good to hear from you, it’s been a while!

Cython translates your Python-code to actual C-code; it converts your mycode.pyx to mycode.c. If you use CythonBuilder this file gets cleaned up automatically unless you use the --no-cleanup flag.

Next Cython converts this C-file to something that your OS can import into Python; on linux-based systems this will result in an .so file, on windows a .pyd file will be generated. This is nothing more that a wrapper that allows Python to call the compiled C-code.

So to answer your question: it is actual C-code! It may be a bit slower because Python has to import the function and then call it but that is a minor overhead.

If your interested: check out this article (in which I create some C-code that gets imported in Python) for more information on how Python handles C-compiled modules.