Comment 1 for bug 1005642

I found a relatively simple workaround:

  STATE=/var/lib/lightdm/.cache/unity-greeter/state
  sudo rm -f $STATE
  echo | sudo tee $STATE

This creates an empty state file owned by root which unity-greeter (running as the lightdm user) cannot overwrite and therefore cannot corrupt. It means the greeter no longer preselects the user who logged in last, but for some people that's a price worth paying. It also provides an easy way to test the bahaviour of unity-greeter to identify what is causing a particular slowdown.

However, the real problem still needs to be fixed and (hopefully) an SRU (stable release update) submitted for 12.04.