It seems that when multiple monitors are attached to a MacBook Pro (Retina), OSX splits up the VRAM for the monitors. Even when a screen is captured by e.g. a game, performance degrades very steep. The only thing that helps is to put the MBP in clamshell mode and remove one of the 2 attached displays to get back to reasonable performance.
The thing I don't understand is, why does the OS not allocate all resources to the captured display by default when it's captured? What's the reasoning behind this?