Any gains in the single threaded model (usually) carry over to the parallel model. And maybe there is a separate effort on making the parallel compiler working better.
I understand that there's a separate effort, but I don't see many blog posts about it, and I see that as something that should be prioritised.
Rust was partly created because most of the transistors on the computers are heavily underutilized, and multiprocessing with C++ correctly is extremely hard.
Rust compiler is written in Rust, so it would be a perfect showcase of taking advantage of the multi-processing safety of the language.
I know that there are global variables in the compiler that the compiler team is getting rid of, but at this point that should be the main focus, as I see that most of the easy huge gains of single-threaded improvements are over.
Maybe they are working on it in parallel.