LLVM might not be the fastest, but when you get to the point that build times become a problem, your code base is too big (or your frontend is doing silly things). Maybe ask your 'automatic code generation' to generate less code bloat ;)
Both should be much faster at compiling debug builds than they are though. There's an LLVM fork (TPDE-LLVM) that supports a limited set of backend targets but compiles way faster (order of magnitude) for O0, but for whatever reason they haven't managed to merge it with the mainline LLVM. Even with that there's still plenty of overhead from all the horrible C++ OOP-brained abstractions LLVM uses.