It turns out it was another issue - the laptop was slow when using a particular charger. I mistakenly assumed it had to do with lightdm because when I disconnected the external screen I disconnected the laptop charger too.
** Changed in: lightdm (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1495206 Title: System slow on dual-monitor To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1495206/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs