The biggest factor in improving LLVM compile/link times is sheer RAM capacity. LLVM compilation is extremely voracious with memory and the more you have the faster it is; otherwise, much time is spent swapping in and out of disk.

I've been compiling it with MSVC on Windows and the swap file after an LLVM compile balloons to ~60 GB. I inserted a couple more sticks to bring my 32 GB to 64 GB and my compile times shortened significantly (I don't have exact stopwatch times right now).