Earlier quoted context omitted.
My understanding is that Cython does not run unmodified standard Python code.
It can run most unmodified Python code, but it needs types for the big speedups.
I think it's far more accurate to say that Cython is a programming language of its own that is a hybrid of Python and C++, that happens to produce CPython extension modules when compiled.
The performance benefits are really achieved by incrementally changing your Python code to something that looks a lot more like C(++).
This is also reflected in the Cython documentation which literally mentions the "Cython language".
Cython is great as an alternative to writing C extension modules for performance reasons or to creating bindings to libraries written in C or C++. It's not so great just to make Python applications faster as it's not fully compatible[1].
[1]: https://cython.readthedocs.io/en/latest/src/userguide/limita...