(ref: https://www.thegoodpenguin.co.uk/blog/reducing-boot-time-with-systemd/)
systemd-analyze time
command provides an overview of the overall system boot time (https://www.freedesktop.org/software/systemd/man/systemd-analyze.html#systemd-analyze%20time)
systemd-analyze blame
This command prints a list of all running units, ordered by the time they took to initialize (https://www.freedesktop.org/software/systemd/man/systemd-analyze.html#systemd-analyze%20blame)
systemctl list-dependencies
Shows units required and wanted by the specified units. (https://www.freedesktop.org/software/systemd/man/systemctl.html)
systemd-analyze critical-chain
This command prints a tree of the time-critical chain of units (for each of the specified UNITs or for the default target otherwise). (https://www.freedesktop.org/software/systemd/man/systemd-analyze.html#id-1.5.6)