But only for operations that someone else has written the C code for you. If you come up with some new algorithm that cannot be reduced to operations already implemented in C, then you would need to write the fast C implementation yourself.

There is a large difference between "implemented in C" (presumably talking about the runtime being implemented in C) and "Python that calls C code". If the implementation of Python is implemented in C as an interpreter#, then the interpreter is almost sure to be slower than whatever language/technology the interpreter was implemented with/on top of. This is pretty orthogonal to "Python that is implemented in <whatever>, and is doing most of the calculations via calling a C library". If you have a Python program that computes over megabytes of data, but it is calling a C library to do the heavy lifting, then the performance of Python itself is probably going to be negligible if 99.9% of the time is spent on the computation in the C code.

# Though could be a VM or something.