I've seen demos of Jonathan Blow's language Jai where he shows how fast his current video game project in Jai compiles. He's doing the opposite of what you say:

I've seen demos of Jonathan Blow's language Jai where he shows how fast his current video game project in Jai compiles. He's doing the opposite of what you say: just give all input files to one compiler invocation without headers or incremental compilation, in order to let it handle concurrency properly. He still ends up compiling at 1mil/s. The added benefit is that his program can get access to the entire AST and modify it during the build.