If this system is not a server in a network, you can start by cutting down on time used by NetworkManager-wait-online.service, to do this please disable the service like so:
sudo systemctl disable NetworkManager-wait-online.service
If it shows up again after reboot, you can follow the above command with this:
sudo systemctl mask NetworkManager-wait-online.service
Pleas, try this first and check if your boot time improves before carrying on with any further steps. This alone, sometimes, reduces your boot time by a great deal.
To roll back the above changes anytime, please run:
sudo systemctl unmask NetworkManager-wait-online.service
Then follow it by:
