Python isnt as slow as you think, if you take care to not do a lot of super intensive stuff constantly or nesting tons of loops inside a main loop. Obviously you cant write an engine like CryEngine in Python, but you can do some pretty nice stuff in Python.
You could additionally write your own pyd's in a lower level language like I do. With swig and some .bat files its actually quite a painless process once you get everything working. Just write anything that is going to be slow, or you need to be really fast (like physics calculations) in something like c/c++ and you have the speed of a lower level language, while still using Python.
The point is, Python is extremely flexible and, at the very least, it can get you a prototype of your game, or help you find out if certain features are going to work/are interesting, with very little headache, and a LOT faster than if you are working only with a low level language.
Edit:
Plus the extremely dynamic nature of Python makes it rock. Not having to declare what type a variable is, and being able to change it on the fly rocks. Dynamic arrays (lists) are also convenient.
Also, to show you the huge difference in c++ and Python, here is a "Hello World" Example for both.
C++
#include <iostream>
using namespace std;
int main ()
{
cout << "Hello World!";
return 0;
}
Python
print "Hello World!"