- the external monitor is connected at boot time, I disconnected it when the login screen is displayed, logged in, no high cpu usage
- logged out, connected the monitor, logged in, HIGH CPU USAGE
- logged out, disconnected the monitor, logged in, no high cpu usage
So, up to and including LightDM's login screen, everything is fine. (The date/time indicator on the login screen pops up the calender immediately for example; when I'm logged into Unity with high cpu usage, the calender only pops up in super slow motion.)
When I log into Unity and the external monitor is connected at the time, I get high cpu usage. When I log into Unity with the monitor disconnected, I do not get high cpu usage. When I connect the monitor once Unity is up, everything works fine, too.
BTW, I also have KDE Plasma Workspaces installed, and I don't see any signs of high cpu usage or graphical slowdowns when I log into that one, even when the external monitor is connected the whole time.
Okay, I did a few more tests...
- the external monitor is connected at boot time, I disconnected it when the login screen is displayed, logged in, no high cpu usage
- logged out, connected the monitor, logged in, HIGH CPU USAGE
- logged out, disconnected the monitor, logged in, no high cpu usage
So, up to and including LightDM's login screen, everything is fine. (The date/time indicator on the login screen pops up the calender immediately for example; when I'm logged into Unity with high cpu usage, the calender only pops up in super slow motion.)
When I log into Unity and the external monitor is connected at the time, I get high cpu usage. When I log into Unity with the monitor disconnected, I do not get high cpu usage. When I connect the monitor once Unity is up, everything works fine, too.
BTW, I also have KDE Plasma Workspaces installed, and I don't see any signs of high cpu usage or graphical slowdowns when I log into that one, even when the external monitor is connected the whole time.