Good god, did people learn nothing from iPhone 4?
When Apple switched to retina displays for iPhone, the amount pixels went up (4 times more pixels) but GPU wasn't 4 times more powerful to handle that - hence iPhone 4 was relatively underpowered in graphics department. Exactly the same thing happend to iPad 3 - 4 times more pixels, GPU not powerful enough.
In both cases follow-ups (iPhone 4S, iPad 4) fixed the problem. You can say that the same thing can even be extrapolated to first gen retina MacBooks (and this is also the reason why I won't buy first gen retina iMac or first gen retina iPad mini).
P.S. This is of course an off-topic. I have no idea why iPad 4 has this issue - perhaps large area of keyboard meant that even iPad 4 couldn't handle it well enough? Don't forget that transparency/translucency is one of the most taxing things in graphics.