2 November 2012The Verge:
The simple fact is that Retina display is a lot of pixels to push around for that graphics chip — just consider that Apple uses the same GPU in the MacBook Air, which has a third of the pixels and can still be made to drop frames under heavy multitasking loads. Plug in an external monitor and you’ll feel the hit come even quicker in day-to-day use — I tried both my 1080p TV over HDMI and my 24-inch 1920 × 1080 display over mini DisplayPort, and I could make YouTube videos stutter on one display simply by loading The Verge in the other.
Not good. I have the 15 inch and it stutters scrolling in Safari, but otherwise it is fine. However, if the thirteen-inch can’t play YouTube videos smoothly â€” that sounds like a bad product.