Using a HDMI cable I have a second screen plugged into my Macbook Pro Retina 15". It's a Dell 27". I've read a lot of discussions about the choppy performance on the Retina and it's true. The performance is choppy on the Retina. But it's smooth on the second monitor. If Mountain Lion is the cause of the choppy performance of the Retina, then why is my second screen smooth (since it's also in Mountain Lion from my rMBP)? This leads me to believe that the pixel density of the rMBP is simply too much for the hardware. If the issue was with Mountain Lion, then surely the second screen would be choppy too.