Comment 7 for bug 1551415

That sounds like it is related. While doing more debug I found that network-manager starts dhclient in forground mode (-d) and that is not affected by bug 1551351. But regardless of how it was started, dhclient seem not to react on SIGHUP. Maybe disconnecting from the network manually uses a more aggressive method of stopping the dhclient than just doing a shutdown or reboot. The system probably goes into the mode of first trying to send SIGHUP to running processes, then wait a bit and finally doing a SIGTERM. That would explain the increased time.