> As an example, in my desktop, it takes ~25min to compile a Debug version of LLVM, but ~23min to compile a Release version.

oh I think I know what might cause this: TableGen. The `llvm-tblgen` run time accounts for a good chunk of LLVM build time. In a debug build `llvm-tblgen` is also unoptimized, hence the long run time generating those .inc / .def files. You can enable cmake variable `LLVM_OPTIMIZED_TABLEGEN` to build `llvm-tblgen` in release mode while leaving the rest of the project in debug build (or whatever `CMAKE_BUILD_TYPE` you choose).