Comment 2 for bug 1627844

I also stumbled over this problem and can provide some results of my experiments:

The problem seems to occur for Ubuntu Vagrant Boxes Bionic and Xenial (Trusty does not seem to be affected).

There is always a problem, when the serial port is either disconnected or disabled. When I redirect the output to /dev/null everything seems to work fine.

Some more details of the behaviour of a Xenial Box:

When Serial 1 Port is enabled the Kernel Boot Process hangs for a while (45 Seconds) after this message was printed:

console [tty1] enabled

After that the Kernel boot process continues very slowly (383 seconds until systemd ist started, given up after 1531 seconds after the journal service was started and nothing more seems to happen). During the process the CPU usage is at about 70% the whole time.

When the Serial port is disabled the behaviour is slightly different:

The Kernel boots normal for about 3 seconds until the usbhid is loaded. Then after 13 seconds the random nonblocking pool is initialized. After that the hard disk indicator for the box is constantly on and nothing more happens.