I've kind of solved it - it's my DVD which is known to have a problem with udev:
This is from an Arch forum, I spent a few hours reading and trying things and my conclusion is that this is authentic (and when I disconnected my DVD it booted super-fast as i would expect):
I've got one of those Samsung SH-223C drives and they are a pain because they don't like the 16-byte pass-through commands that udev uses (search your logs for >"IDENTIFY PACKET DEVICE" errors). The fix has been posted here several times: Add
options libata atapi_passthru16=0 force=5:udma5
to /etc/modprobe.d/modprobe.conf, add
to /etc/mkinitcpio.conf, rebuild the kernel image with mkinitcpio and reboot.
I would like to be able to re-build the kernel to check / solve this but I can't find out how so I've added a bug report https://dev.solus-project.com/T5762
and as it's only me I may just buy a new DVD, and in the meantime I use sleep so it's not really a bother.