Intro.

Ubuntu checks network configuration on its startup. Although this process passes quickly when the network connection is normal, it takes about 5 minutes if there exist some problems in the configuration.
There is a way to reduce the waiting time for the network configuration.

Reference

Ubundtu forum answer

Solution

You can change the value of the waiting time. Set the value of TimeoutStartSec in the file, /lib/systemd/system/networking.service.
For example,

...
TimeoutStartSec=15sec
...

My system was Ubuntu 16.04, FYI.

Good luck! :D