Using anything above 62 with the -j option causes the qbs process to wait indefinitely at some point during the build. The warning "QWinEventNotifier cannot handle more than 62 handles" is printed out. QTBUG-8819 shows that it is a windows limitation but qbs should be able to handle it.

Edit: Sorry, my mistake. Just realized this is already fixed in Qt 5.10. However, Qbs still uses Qt 5.9.3, so any plans on updating it?