What's wrong with js for neural networks? JS/V8 is faster than python, not accounting for python bindings to native NN libs that may as well be bound to js.
JS is faster than Python, but in the land of DL, there is C++ and everyone else. No one is using Python to do the actual computation anyway.
Depends what "actual" computation is. If you're definition of "actual computation" is something which requires extreme optimization then your definition precludes the question. In the real world there is an extraordinary amount of computation done with inefficient languages though simply because development time costs very often outweighs run time costs.
The point is that none of computations are done in pure Python. Python just provides a convenient wrapper over non-Python code.