> The example you gave of a compiler is canonically implemented as multiple process making .o files from .c files, not threads.

This is a huge limitation of C's compilation model, and basically every other language since then does it differently, so not sure if that's a good example. You do want some "interconnection" between translation units, or at least less fine-grained units.

And yet despite that theoretical limit C compiles faster than any other language. Even C++ is very fast if you are not using header-only style.

What’s better? Rust? Haskell? Swift?

It’s very hard to do multithreading at a more granular level without hitting amdahl’s law and synchronization traps.