Python extension modules written in C are faster than the equivalent programs written in pure Python. How do these extension modules compare (speed wise) to programs written in pure C? Are programs written in pure C even faster than the equivalent Python extension module?
Being a Python extension doesn't affect the execution speed of a piece of code, except insofar as the Python invoking it is slower than the equivalent C would be, and the compiler is less able to aggressively unroll and inline code which crosses the C/Python boundary.
That is to say, if you just have Python code call a C function, and then you do all your work in that function, the only performance difference is going to be the amount of time you spent before getting into the C side of things. From that point on, it is native C.
Speed of Python Extensions in C vs. C