Extremely slow Kubuntu Boot (18.04 LTS)

A few services can be disabled to reduce the boot-up time with no effect on the desktop environment functionality in most cases:

Firstly:

POSTFIX appears in your post. If your machine is not a mail server and you do not use POSTFIX you can disable the postfix.service by running the following command in the terminal:

sudo systemctl disable postfix.service

Or you can uninstall POSTFIX all together by running the following command in the terminal:

sudo apt remove postfix

Secondly:

NetworkManager-wait-online.service can be disabled by running the following command in the terminal:

sudo systemctl disable NetworkManager-wait-online.service

Please refer to this answer for more explanation.

Thirdly:

Two services that rely on the functionality of NetworkManager-wait-online.service are networkd-dispatcher.service and systemd-networkd.service. These can be disabled as well by running the following commends in the terminal:

sudo systemctl disable networkd-dispatcher.service

And:

sudo systemctl disable systemd-networkd.service

These two services are used strictly for systemd's connectivity purposes and will not affect the connectivity of the desktop environment after login.