I just had time to make a quick test of this and, as far as I can tell, the -j0 option isn't working, i.e. there is no noticeable change in wall time using time make install. If I change to, say, -j3, there is definitely a modest speed up. Tested briefly on both macOS and Linux. On both systems os.cpu_count() > 0. I don't have time to look further at it at the moment.