Why does my Mac have laggy/choppy Dock magnification effect?
Unfortunately, a lot of the graphics on the MacBook Pro with Retina Display are a bit choppy (for example, scrolling and window resizing). The new rMBP simply has a lot of pixels, and the graphics card is no better than the one in the regular MBP. In the next few years we can probably expect this to improve with further software optimizations and better hardware offerings.

See also:


Retina MacBook Pro Pushes the Limits of its Graphics Capabilities (MacRumors)
Driving the Retina Display: A Performance Discussion (AnandTech)