> Why is “developers might have to interact with it” some kind of non-starter, as though having a compile phase is a worse evil than a hyper-slow language?

For big monoliths (like ours at IG), the server start-up can take more than 10sec, which is already super high for a "edit -> refresh" workflow. Introducing a Cython like compilation step is really a major drawback for every single developer.

For smaller projects, Cython works extremely well (and we do use it for places where we need to interface with C/C++).