Hi all, I recently ran into this problem on my KVM host after I migrated all my VMs to use unpartitioned virtual disks for everything except the /boot disk.
A little investigation revealed that the problem was related to pvscan processes started by the 69-lvm-metad.rules udev script. At the end of that script there is a conditional check that results in the two branches "systemd_background" and "direct_pvscan" (lines 92 and 93). The error case uses the "direct_pvscan" branch and results in the delays. Changing the script so that the "systemd_background" branch is always used instead (just add a 'GOTO="systemd_background"' line before line 92) completely solves the delays for me, both on virtual and physical systems. I also tested online hotplug and did not notice regressions with the change. Reverting the change from line 475 in the debian-patches file (https://sources.debian.org/src/lvm2/2.03.07-1/debian/patches/debian-changes/#L475) however does not bring any improvement. I hope this helps anyone running into this problem. Regards Oliver
