In Rust's crate compilation model, there's certainly some unexploited parallelism. Often as much as half of the compilation is spent in the LLVM phases. By then, all the MIR is already around and only sitting there, waiting on LLVM to finish. Downstream crates could already start their compilation with the MIR data only. Only the LLVM phases of the downstream crates need the LLVM data of the upstream crates. Assuming that half of the time is spent in MIR, half in LLVM IR, you would be able to double your parallelism, or halve the length of the critical path through the compilation graph.

Yes, often things are compiled in parallel but often there are tight spots in the compilation graph where only one crate is compiling because all later crates are relying on it.