The codegen is. This is partially because the IR passed to LLVM is not the best, but it’s also not terrible.

Does Rust allow for fast unoptimised builds then? Perhaps I'd missed that.

Faster, yes. Still lots to do.

You're saying LLVM is slow even when generating code without optimisations?

Yes.

The thing that will massively speed up rustc is the current re-architecting of it. We’re at the point of “few percent here, few percent there” with the current design. These add up over time, of course, but batch compilers are inherently slower than the newer style ones (after an initial compile).

To add to Steve’s point, Rust adds more checks during debug (non-release) more, which can makes the IR larger. So it’s not always the case that debug mode is faster to compile (though it generally is)