I have Ubuntu 16 running on a Raspberry Pi 3 mobile platform, and if it's not connected to ethernet, and the wifi's out of range during boot, it takes about 15 minutes to boot, since it appears Ubuntu has a very long timeout while it waits to bring up eth0 or wlan0. I've tested this repeatedly, and when the boot takes forever, I find plugging in an ethernet cable magically makes boot completely instantly.
How do I disable this, so Ubuntu boots up quickly, regardless of network connectivity? I'm using Network Manager, which is excellent at re-establishing a wlan0 connection onces an recognized SSID is detected.
/etc/network/interfaces
for eth0 from auto to manual also stopped the hanging during boot.