Glad you asked! We're trying to find good ways to help people investigate this. One of my colleagues wrote up on his work on TypeScript 4.1's `--generateTrace` flag: https://github.com/microsoft/TypeScript/wiki/Performance-Tra...

That page can give some guidance on diagnosing what the compiler's time is going into. Most users won't need to do this, but there are plenty of bigger teams/organizations that are really invested into TS and do want to keep their build times lean.