The performance on secondary displays is abysmal in Parallels 8. Dragging a window is fast on the main display, but as soon as you get to a secondary monitor the performance goes down to about 10-20 FPS (depending on hardware). On my Retina MBP it is slowest at 10 FPS. On my Mac Pro I would estimate to about 20 FPS. Also when you drag a window between monitors the mouse pointer snaps to the wrong location. When you release the mouse button the Window snaps back to the original monitor. This is on 10.8 with Windows 8. There is also a problem with mixing retina and non-retina displays where the position of the displays become incorrect, and obviously you cannot get Windows to display correctly as you can only set one DPI for all displays on Windows. This should be fixed by parallels adding an option to double the resolution on non-retina displays and scale it back to the original resolution. So a 2560x1440 display would be rendered at 5120x2880 and scaled down again to 2560x1440.