RubyMotion spawns as many compiler processes as possible to speed up compilation. In your case you probably have a quad core machine, which translates to 8 cores thanks to HyperThreading. You can check this by running:
/usr/sbin/sysctl -n machdep.cpu.thread_count
The time it takes to compile depends on how many Ruby files your project has, and how big your depencendies are.
If you can put together a test project that takes unreasonably long to compile, please upload it to GitHub and I will take a look