The flag is set to do the nightly build. See https://github.com/Slicer/DashboardScripts/blob/4efec4e330f4e79da9d9eef3ac445a350bde4618/overload-vs2013-slicer_release_nightly.cmake#L57
Since having four cores (or more) is fairly common, may be we could make this the default ?