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.
You can change the value of the waiting time.
Set the value of
TimeoutStartSec in the file,
... TimeoutStartSec=15sec ...
My system was Ubuntu 16.04, FYI.
Good luck! :D