Don't people think this is one tool that would greatly benefit from using the very fastest languages available? Where's the C++, Rust, maybe even FORTRAN version?

This is only the language for describing the volumes. That's not heavy, rather the importance is that you can express the ideas you want. The heavy lifting of rendering and computing how volumes interact etc is already implemented in native code.

My understanding is that the core of OpenSCAD is done using C++.

If you wish to use Rust for 3D modeling directly there is:

https://fornjot.app/

(the developer of which is actually working on a BREP kernel)