Python's supporters will tell you that its slowness doesn't matter, because you can re-write the time-consuming bits of your code in C. I can imagine that that works well for web programming where latency accounts for much of the execution time anyway and one might only occasionally have a really chunky calculation to do. But in my experience doing Monte Carlo simulations, solving large systems of non-linear equations and that type of thing, a large part of the code is fairly critical for execution speed, so re-writing in C would mean doing a large part of the project in C. If I wanted to do that, I'd just write in C to begin with.