I installed a minimal Ubuntu server image on a little Toshiba NB100 netbook I was given as a present and there was an annoying side effect whereby if a wireless network wasn’t detected on boot the bootup process would wait for about 5 minutes looking for a network before moving on. This happened every single time I booted up because I preferred to only search for wireless networks manually. The answer was to edit the networking.service config file:
sudo systemctl edit networking.service
Then enter into the file:
Service
TimeoutStartSec=20sec
to set the timeout period when looking for wireless networks to be 20 seconds.
Check the file has the correct information:
systemctl cat networking.service