When you reach the cryptsetup prompt, press ESC, then insert your decryption password as you’d normally do, just without plymouth. This way you’ll be able to see which processes take the longest.
Alternatively, if you’re already in dom0, type the following in a terminal to verify the exact times for each process:
$ systemd-analyze blame
If you have an Intel CPU you may be suffering from a known bug: here’s how you can resolve it: System unusable without nomodeset - #3 by BEBF738VD
Have you tried kernel-latest? Unable To Complete Login With New GPU - #6 by BEBF738VD