Comment 31 for bug 1309617

I was having this problem with plymouth-upstart-bridge respawning repeatedly during boot. Machine still came up, but boot seemed to be delayed.
Here's a very simple solution I found here: http://www.unrelatedshit.com/2014/07/30/kvm-too-fast-for-plymouth-upstart-bridge/

To fix it, simply add a "sleep 2" to your /etc/init/plymouth-upstart-bridge.conf like so:

[...]
stop on (stopping plymouth
         or stopping plymouth-shutdown)

console output

exec plymouth-upstart-bridge
sleep 2