[kelnos]

I doubt it's actually calling out to the CLI (aka the shell); presumably it's just fork()ing and exec()ing.

On Linux, fork() is actually reasonably fast, and if you're exec()ing a binary that's fairly small and doesn't need to do a lot of shared library loading, relocations, or initialization, that part of the cost is also fairly low (for a Rust program, this will usually be the case, as they are mostly-statically-linked). Won't be as low as crossing a FFI boundary in the same process (or not having a FFI boundary and doing it all in the same process) of course, but it's not as bad as you might think.