> The best way to speed it up appears to be organizing a codebase in many crates.

A "crate" in Rust is the unit of compilation. In C, a file is the unit of compilation. Rust just lets you have a compilation unit that's composed of more than one file (without having to resort to C-style textual inclusion). But if you want, you can certainly have one-file-per-crate, just like you would in C. And what's nice about having many crates is that crates forbid circular dependencies, which trivially enables coarse-grained parallelism in the build system. So yes, organizing a large codebase into crates is the best way to achieve parallelism, but that isn't something to be deplored (and strictly controlling circular dependencies is useful for comprehending large codebases in general).