I thought Python offers an entry into the performance space via Cython, which seems to nearly map directly to C. Is Cython not a viable alternative to Julia?

I wouldn't say so. Cython is quite literally C for python, basically making all your python code look like C with most disadvantages, while not always reaching C performance.

Julia in comparison is a fully featured language, with a lot to offer - you can write highly generic + fast code in a very high-level way. E.g. have a look at https://medium.com/@Jernfrost/defining-custom-units-in-julia... You can see, that Julia can be more elegant than python while being a lot faster.

One of the main issues is that, because you compile things separately with a context-switch managed by python in the middle, if you pass a Cython compiled function to code that is calling compiled code (C/Fortran/etc.), then you still get a huge overhead. We tested this with ODE solvers, and things like Numba+SciPy odeint were still about 10x slower than they should be because of this phenomena ([1] mentions some of our tests). In the end, we found a very good reason to make sure the whole stack can compile together!

[1]: http://juliadiffeq.org/2018/04/30/Jupyter.html

Yes, this is aimed at the same people, I believe. Doodle in Python and then work harder on the critical part.

I think this works best when you have some loops pushing arrays of Float64 around. And worst if you'd like to pass some library a million little functions to optimise which depend on some strange data type you just defined... Cython is quite a limited sub-language. But certainly many current Julians made a living this way in the past.