Ubuntu 15.04 network manager causing slow boot

I faced the same problem before, NetworkManager taking about 8 seconds. There are three ways to solve this problem:

1. Disable connecting automatically for your connection

   edit connections > select your connection > edit and disable connecting automatically

   

2. You can simply disable Enable networking from network indicator.

3. You can disable the service by running this command

   systemctl disable NetworkManager-wait-online.service
   

You can change the timeout of nm-online to something like 10:

Edit /lib/systemd/system/NetworkManager-wait-online.service as root or using sudo.

Look for the lines:

[Service]
Type=oneshot
ExecStart=/usr/bin/nm-online -s -q --timeout=30

Change 30 to whatever you like. 10 worked well for me.

But beware this may break services that depend on it.

Source

Disable dhcp on eth0 interface.

If you have dhcp configured on eth0, and the wire is not connected, the system will keep looking for an ip address to assign your device, until the request for ip address times out, before the boot continues.

To verify if this would solve your problem, do sudo /etc/init.d/networking restart and take note of how long it takes for this service to restart.

The service will restart in no time, if there's no issues around eth0 interface, else, a long delay and timeout will be presented.