For anyone else having the same issue:
I just installed an ssd and put my Mint installation on it (timeshifted a backup over a new installation, changed some UUIDs in fstab). Expected much faster boot times, but didn't get it. Was around 1 minute from hdd before, and is around 1 minute with ssd now. The command systemd-analyze displayed 38.808s for loading the kernel.
Found a thread from 2018(!) where this behaviour was discussed in the context of Lubuntu. Turns out, I needed to create a file /etc/initramfs-tools/conf.d/resume with content:
RESUME=none
(Apparently this disables hibernation. Never used it anyway.)
Updated initramfs, rebooted, any now total boot time is around 30 seconds, and kernel boot time now was 5.972s. Big difference! Maybe this file was created with the new installation and it was deleted by my timeshift restore. Anyway, worth a shot if you experience the same issue.