Open up your Preferences->Build Options and make sure that the “maximum worker threads” and “maximum builds at once” are both set to the number of CPUs you want to use. Also make sure “Allow devices to multithread when available” is checked.
At the bottom, you can turn off “use low priority build thread” to elevate WM’s build process priority. This will cause other applications on your system to respond more slowly, but increase the time slices devoted to WM.
What kind of CPU are you using? If it is 8 core due to hyper threading or something similar, you may not see full utilization of the extra cores.