[targafarian]

Why not just write entire project in (pick a language).

Sometimes you have a Python project already, you aren't planning to write a new project. In fact there's a ton of non cython python in existence, every time I use a new library, your solution is to rewrite it in cython?

Lots of ways to speed up python. It's great to have this option, which will work for some without having to change much of anything. Cython, Numba, C (etc.) extensions, ... All are good, too, where they fit into a project and development cycle.