Giving the compiler a higher priority is a really, really stupid idea. It will not make the compilation as much as a second faster, but it will severely hurt your system's responsiveness. Don't do that.
Giving the compiler a lower priority may make sense if you have a low-end computer and still want to be able to surf the internet (or do other things) while a lengthy compile is running. However, you probably don't need that unless your computer is 6-7 years old. Multicore CPUs work quite well in that situation without any priority tweaking, even the cheap ones.