The FAQ states it is often much slower:

> We have support for c-extension modules (modules written using the C-API), so they run without modifications. This has been a part of PyPy since the 1.4 release, and support is almost complete. CPython extension modules in PyPy are often much slower than in CPython due to the need to emulate refcounting. It is often faster to take out your c-extension and replace it with a pure python or CFFI version that the JIT can optimize.

https://doc.pypy.org/en/latest/faq.html#do-c-extension-modul...

I have seen great success with cffi though.