Ubuntu boot very slow with wifi driver

[pfeerick]

The fix is that you change the configuration to  match your use case.

Indeed. If you want the ethernet to not black startup when the cable is not connected, you'll need to either disable it, or configure it to allow for hotplugging. 

Depending on your network setup, this may be as simple as changing /etc/network/interfaces to read 'allow-hotplug eth0' instead of 'auto eth0'. This allows the system to configure the device it if is plugged in later. 

In my case, this means the part of my /etc/network/interfaces file that deals with the wired ethernet looks like the below snippet, and setups up my ethernet connection when and if the cable is plugged in. 

Code:

#auto eth0 #commented out to prevent blocking system startup if cable not plugged in
allow-hotplug eth0
iface eth0 inet dhcp

If you have a more complicated network setup, you may find you need to consider using the systemd-networkd daemon, but in my case that wasn't needed.