So I've done some limited work with writing C/numpy extensions (for numerical work) and it's gone swimmingly. However, a lot of performance-critical things I need to do don't necessarily fit into array/matrix numerical computing - lots of times there are difficult,complex datastructures involved that need to be accessed in the "inner loop" (not really a loop, but you get the point). I typically end up using Java for these sorts of problems, although sometimes Pypy does okay.

I rather dislike the constant advice I see doled out to those who mention Python's performance issues - namely, that they can just rewrite the performance critical parts in C. Lots of the time, the performance critical parts are just as sophisticated and require just as much abstract, high-level coding as the rest of the program. Some awareness of that would be nice.