I remember a project that used boost for very few things, but it included a single boost header in almost every file. That one boost header absolutely inflated the build times to insane levels.

I once moved a C++ project to C, and compile times went from 15 minutes to 5 seconds. It was a huge productivity boost. LLVM being slow to compile is also one of the reasons the Zig folk are looking to make it an optional dependency.