Comment 76 for bug 1222602

For your crt/lcd monitor -> maybe this issue depends on the resolution of the external screen.

Does appear that issue if you have connected your screen at boot-time, then disconnected when the lightdm login screen is showing and you logged in (without connected external screen)?

Today I had the problem again. Booted and did login (external monitor connected over vga but not used for output)-> high compiz cpu usage
logged out and in again -> high cpu
disonnected monitor, logged out, logged in -> normal compiz cpu usage
again logged out/in -> normal compiz cpu usage
connected monitor again, logged out/in -> high compiz cpu usage again (still the external monitor is not used for output)