After upgrading to Yosemite, I'm seeing very high CPU usage from WindowServer (typically over 50%). On a 2010 MB Pro 2.66/8GB RAM/SSD. Load was under 1.5 typically on Mavericks - now it runs hot and sluggish. Killed off all apps that I thought could be related - no real change. I am running an external 2nd monitor, if that matters. Anyone else have this problem? ** Update , it seems having a second display is causing this. When I unplug the 2nd display, the problem seems to go away (may be too early to tell). I'm using an Accell DisplayPort adapter which has always treated me well.