Ah, yes, I knew I had to be missing something! Needing to support incremental builds are what makes C++ compilation so frustrating. I wonder why this is so difficult though - dependency tracking should be a thing that can carry through the link stage. Just track what files a given function depend on, and only recompile/recodegen functions that have changed. Of course, it still sucks massively if you change a header file, but that's why you separate the header from the implementation of the methods :)

/LTCG also doesn't seem to parallelize well - last I checked it still ran all the codegen on one core. Maybe that's different now?