I have a desktop with an Asus 4870 graphics card and two displays hooked up (a 23" Samsung SyncMaster 2333 and a 17" iiyama ProLite E430) with extended desktop. However, I am having severe performance issues. With only one display plugged in, Ubuntu feels smooth and zippy like it should, regardless of further configuration. But when two displays are plugged in, the performance drops horribly. Everything just feels slow and laggy, regardless of desktop environment (Unity and GNOME) or drivers (open source FGLRX, drivers from the AMD website and no drivers at all).
Has anyone experienced similar problems? And does anyone know how to fix this?