Since parallel boot happened, during boot the cryptsetup password prompt in my system gets flooded with other boot messages.
I fixed it, as suggested in #764555,
installing plymouth, then
editing /etc/default/grub to add splash to GRUB_CMDLINE_LINUX_DEFAULT:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
Besides showing pretty pictures (and most importantly, getting them out of my
way if I press ESC), plymouth also provides a user prompt that works with
parallel boot
which sounds like what I needed.