In the Zig stage1 compiler (written in C++), I tried to limit all the C++ headers to as few files as possible. Not counting vendored dependencies, the compiler builds in 24 seconds using a single core on my laptop. It's because of tricks like this:

    /*
     * The point of this file is to contain all the LLVM C++ API interaction so that:
     * 1. The compile time of other files is kept under control.
     * 2. Provide a C interface to the LLVM functions we need for self-hosting purposes.
     * 3. Prevent C++ from infecting the rest of the project.
     */


    // copied from include/llvm/ADT/Triple.h

    enum ZigLLVM_ArchType {
        ZigLLVM_UnknownArch,
    
        ZigLLVM_arm,            // ARM (little endian): arm, armv.*, xscale
        ZigLLVM_armeb,          // ARM (big endian): armeb
        ZigLLVM_aarch64,        // AArch64 (little endian): aarch64
    ...

and then in the .cpp file:

    static_assert((Triple::ArchType)ZigLLVM_UnknownArch == Triple::UnknownArch, "");
    static_assert((Triple::ArchType)ZigLLVM_arm == Triple::arm, "");
    static_assert((Triple::ArchType)ZigLLVM_armeb == Triple::armeb, "");
    static_assert((Triple::ArchType)ZigLLVM_aarch64 == Triple::aarch64, "");
    static_assert((Triple::ArchType)ZigLLVM_aarch64_be == Triple::aarch64_be, "");
    static_assert((Triple::ArchType)ZigLLVM_arc == Triple::arc, "");
    ...

I found it more convenient to redefine the enum and then static assert all the values are the same, which has to be updated with every LLVM upgrade, than to use the actual enum, which would include a bunch of other C++ headers.

The file that has to use C++ headers takes about 3x as long to compile than Zig's ir.cpp file which is nearing 30,000 lines of code, but only depends on C-style header files.