After writing that, I wrote my own standard library (it has data structs like vector, hashmap and sets; slices, strings, rng, print, some io functions, and more) which uses a lot of templates, and it compiles in <200ms on both clang and gcc. Many standard library headers take much longer to compile than that. It's not a terrible idea to have your own standard lib if you need quick compile times.
Sad but not surprised to see it's no longer maintained (8 years ago!).
Even in the era of terabyte NVMe drives my eyes water when I install MSVC (and that's usually just for the linker!)