You can remove plymouth during your boot up process and display progress messages by using:

sudo -H gedit /etc/default/grub

Find the line containing:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

and remove quiet splash between the double quotes. If you have additional parameters leave them as is.

Save the file and run:

sudo update-grub

Then reboot and watch the messages that appear for any clues on what is causing delays.

After boot completes run:

systemd-analze blame

once more and report back with the results.