In reply to @lily:lily.flowers
Would you mind nabbing logs with plymouth:debug on the kernel cmdline sometime?

$ journalctl -b 0 | grep -i plymouth
Jun 15 15:31:59 localhost kernel: Command line: initrd=\efi\nixos\xdmzixriby5jk6jxvd3m79ciryndjnfm-initrd-linux-6.3.7-initrd.efi init=/nix/store/qr8c7ccym056dricl45qlc9kni6w4r8b-nixos-system-jace-23.11.20230615.c5ca97a/init delayacct quiet nohibernate splash loglevel=0 plymouth:debug
Jun 15 15:31:59 localhost kernel: Kernel command line: initrd=\efi\nixos\xdmzixriby5jk6jxvd3m79ciryndjnfm-initrd-linux-6.3.7-initrd.efi init=/nix/store/qr8c7ccym056dricl45qlc9kni6w4r8b-nixos-system-jace-23.11.20230615.c5ca97a/init delayacct quiet nohibernate splash loglevel=0 plymouth:debug
Jun 15 15:31:59 localhost kernel: Unknown kernel command line parameters "splash plymouth:debug", will be passed to user space.
Jun 15 15:31:59 localhost kernel:     plymouth:debug
Jun 15 15:31:59 localhost systemd[1]: Tell Plymouth To Write Out Runtime Data was skipped because of an unmet condition check (ConditionPathExists=!/etc/initrd-release).
Jun 15 15:31:59 localhost systemd[1]: Starting Show Plymouth Boot Screen...
Jun 15 15:31:59 localhost plymouthd[373]: 00:00:00.496 ../src/main.c:1956:check_verbosity                            : redirecting debug output to /dev/tty1
Jun 15 15:31:59 localhost plymouthd[373]: xkbcommon: ERROR: failed to add default include path /nix/store/0zl3x9ggy4sdik97cgbijkalllmkw70b-xkeyboard-config-2.33/etc/X11/xkb
Jun 15 15:31:59 localhost plymouth[374]: 00:00:00.507 ../src/libply/ply-event-loop.c:766:ply_event_loop_stop_watchin: stopping watching fd 6
Jun 15 15:31:59 localhost plymouth[374]: 00:00:00.507 ../src/libply/ply-event-loop.c:782:ply_event_loop_stop_watchin: removing destination for fd 6
Jun 15 15:31:59 localhost plymouth[374]: 00:00:00.507 ../src/libply/ply-event-loop.c:766:ply_event_loop_stop_watchin: stopping watching fd 6
Jun 15 15:31:59 localhost plymouth[374]: 00:00:00.507 ../src/libply/ply-event-loop.c:782:ply_event_loop_stop_watchin: removing destination for fd 6
Jun 15 15:31:59 localhost plymouth[374]: 00:00:00.507 ../src/client/ply-boot-client.c:807:ply_boot_client_detach_fro: detaching from event loop
Jun 15 15:31:59 localhost systemd[1]: Started Show Plymouth Boot Screen.
Jun 15 15:31:59 localhost systemd[1]: Dispatch Password Requests to Console Directory Watch was skipped because of an unmet condition check (ConditionPathExists=!/run/plymouth/pid).
Jun 15 15:31:59 localhost systemd[1]: Started Forward Password Requests to Plymouth Directory Watch.
Jun 15 15:32:03 localhost systemd[1]: Started Forward Password Requests to Plymouth.
Jun 15 15:32:07 localhost plymouthd[373]: Fontconfig warning: no <cachedir> elements found. Check configuration.
Jun 15 15:32:07 localhost plymouthd[373]: Fontconfig warning: adding <cachedir>/var/cache/fontconfig</cachedir>
Jun 15 15:32:07 localhost plymouthd[373]: Fontconfig warning: adding <cachedir prefix="xdg">fontconfig</cachedir>
Jun 15 15:32:10 jace systemd[1]: Starting Plymouth switch root service...
Jun 15 15:32:10 jace systemd[1]: Finished Plymouth switch root service.
Jun 15 15:32:10 jace systemd[1]: systemd-ask-password-plymouth.service: Deactivated successfully.
Jun 15 15:32:10 jace systemd[1]: Dispatch Password Requests to Console Directory Watch was skipped because of an unmet condition check (ConditionPathExists=!/run/plymouth/pid).
Jun 15 15:32:10 jace systemd[1]: plymouth-switch-root.service: Deactivated successfully.
Jun 15 15:32:10 jace systemd[1]: Stopped Plymouth switch root service.
Jun 15 15:32:11 jace systemd[1]: Starting Tell Plymouth To Write Out Runtime Data...
Jun 15 15:32:11 jace plymouth[1643]: 00:00:12.595 ../src/libply/ply-event-loop.c:766:ply_event_loop_stop_watchin: stopping watching fd 6
Jun 15 15:32:11 jace plymouth[1643]: 00:00:12.595 ../src/libply/ply-event-loop.c:782:ply_event_loop_stop_watchin: removing destination for fd 6
Jun 15 15:32:11 jace plymouth[1643]: 00:00:12.596 ../src/libply/ply-event-loop.c:766:ply_event_loop_stop_watchin: stopping watching fd 6
Jun 15 15:32:11 jace plymouth[1643]: 00:00:12.596 ../src/libply/ply-event-loop.c:782:ply_event_loop_stop_watchin: removing destination for fd 6
Jun 15 15:32:11 jace plymouth[1643]: 00:00:12.596 ../src/client/ply-boot-client.c:807:ply_boot_client_detach_fro: detaching from event loop
Jun 15 15:32:11 jace systemd[1]: Finished Tell Plymouth To Write Out Runtime Data.
Jun 15 15:32:11 jace systemd[1]: Dispatch Password Requests to Console Directory Watch was skipped because of an unmet condition check (ConditionPathExists=!/run/plymouth/pid).
Jun 15 15:32:16 jace plymouth[2047]: 00:00:17.568 ../src/client/plymouth.c:425:on_disconnect                    : disconnect
Jun 15 15:32:16 jace plymouth[2047]: 00:00:17.568 ../src/libply/ply-event-loop.c:1143:ply_event_loop_free_destin: freeing destination (0, (nil), 0x403d30) of fd 6
Jun 15 15:32:16 jace plymouth[2047]: 00:00:17.568 ../src/client/ply-boot-client.c:807:ply_boot_client_detach_fro: detaching from event loop

In reply to @elvishjerricco:matrix.org
since it's changing the default of an existing option

In reply to @elvishjerricco:matrix.org
I believe the gap between Started Show Plymouth Boot Screen. and Fontconfig warning: no <cachedir> elements found. Check configuration. is the gap where nothing is happening. I see some of these plymouth debug messages on screen, and then it stays like that for about 8 seconds, and then the password prompt suddenly shows up.

In reply to @elvishjerricco:matrix.org
So the message can calculate which mountpoints are busted

https://github.com/NixOS/nixpkgs/pull/238021

I punted on the calculation for now, as much as I'm in favour of better errors for nixos in particular. But the message text seems sufficient