I have recently experienced many problems when trying to learn digital painting with Photoshop and a Wacom tablet. Everything works fine if I use an external monitor, but becomes supper laggy if I use the retina screen on the actual laptop, due to the significantly higher resolution. My MBPr only has Intel Iris and I guess it just can't render stuff fast enough on such a high-res display. Is there a way to reduce the resolution of a retina display? Not scaling, but actually reducing the number of effective pixels... Thanks!