blender uses python which is an interpreter language. it is not like c++. there is no claim about it is slow. what makes blender fast?
What makes you think blender is programmed just in python ?
Python, as it is used in blender, is basically the interface to a core which is written in C.
Python can deal well with the simple things that perhaps set or return a few values, up into the tens or even hundreds of repetitions. You wouldn’t notice a lag whether it was Pascal, C, Python, or even brainfuck for that matter, because ultimately, its only a few things and our computers are fast. (near identical, i should say 
A lot of times, the only things getting moved around are the names of the things, and not the values of the things.
C is better suited for those calculations which can run up into the millions or repetitions, which need to take advantage of the hardware, language optimizations, etc…
Python can be slow , super slow, fast, blazzing fast. Depending on how good coder you are. The same applies for C and C++.
No programming language is either “fast” or “slow”. Because the speed can vary ALOT from example to example and depend on millions of factors.
Python is written in C.
Python slowness can be because of bad coding , or because of dynamic types.
Python code that wraps C code will be as fast or very close to the speed of C code.
Python speed can vary alot from implementation to implementation. Blender uses the Cpython implementation, the pypy implementation can be even 100 times faster in some cases.
Cython is a python implementation that can produce python code that can run as fast and in some cases faster than C. Cython converts python code to c which then compiles it.
Libraries also can deliver super speed, python is very popular for intense computations unlike what the earlier poster claims, there is no reason to use C, when you can use libraries like Scipy and Numpy which are used heavily as Matlab replacement.
Its possible to write Blender completely in python and make it run even faster.