No, in practice is works something like this:

1) write everything in python

2) yeah, the performance here is good enough so ship it

3) there is no 3

There are very few situations where performance is going to be an issue for you where there is not an existing C module solution that will solve the problem for you. The tired old 'python is slow' trope is getting more and more irrelevant every day. There are other aspects of the language that may make it a mediocre solution to the problem at hand, but out in the real world most people are simply getting the job done with python.