I think that is a reason why there's a lot of python/c++ in hedge fund land. I've written some clojure but don't know the c interop story for it.
You just opened the box of pandora for a million reasons most of which are unbeknownst to the both of us :) But for the sake of argument I will continue under this assumption
> language to be about as fast as possible
If you're interested in Mathy stuff like Machine Learning, FFT, ... then maybe. But even for those you usually have JNI bindings, so it's easy to use most of those mathy C libs if necessary.
But I guess that 95% of all software isn't about speed but about something else (correctness, maintainability, safety against threats, portability, ...) because costs today are usually dictated by manpower costs or those arising from safety/security incidents and much less often by hardware costs compared to say a decade ago.
The Python libraries never seem to care. Just give them a list of a list of number-ish values and off they go.
Java might have some great ML/Math libraries, but the fact that Python dominates data science suggests that my experience is a real-world pain-point.