JS/V8 may beat Python in general operations but definitely not in math specialized libraries. A lot of python packages support OpenBLAS and AVX/FMA by default which gives huge boosts to math/tensor operations. Correct me if I'm wrong, I don't think the js ecosystem is quite there yet except for experimental projects.

There is nothing wrong to use js, like there is nothing wrong to call it impractical.

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.

"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.

Python is used because there are C/C++ bindings for working with NNs.

The point is that none of computations are done in pure Python. Python just provides a convenient wrapper over non-Python code.