I think the thing that stops me from running more processes inside emacs is that a large diff parse from magit or similar can block everything, and so I end up keeping a separate terminal around anyway.
Huh is that true? Does running a command in a terminal buffer block all of EMacs? That's extremely surprising, the main use of these kinds of integrated terminals for me is to compile and run which means long running commands. Is this use case fundamentally not supported due to EMacs's architecture?
I'm confused, what emacs do you have in mind? For me, emacs is a lisp machine, hackable live, M-. all the way down, that kind of things, that's the whole point (again, subjective opinion, and I'm interested in yours).
What language do you have in mind?
If you're worried about performance, Common Lisp is a compiled language. It's about the best performing dynamic language there is, and not too far behind Zig, Rust and the likes.