Apparently setting the lvm2-monitor service to start "manually" and possibly stopping the lvm2-lvmtad and lvm2-lvmpolld services seems to be enough to solve the problem. Done 10 reboots without problems, I would have seen 6 or 7 delayed reboots with the default services config. BTW, updated to the current released version in the meantime. lvm as the culprit seems to be confirmed by at least one other user, see: https://forums.opensuse.org/showthread.php/536145-OpenSuSE-Leap-15-1-shutdown-reboot-delay-of-90-seconds