Fix lagging display performance on retina MacBook Pro

This weekend my Mac suddenly started behaving strangely: moving windows around occurred with a nearly psychodelic delay, mission control (aka exposé) was “jerky”, and scrolling was not fun. Forcing the graphics card to the discrete NVIDIA GT 650 instead of integrated Intel GPU sped things up, but the overall experience still didn’t feel right. Since the onset was sudden I immediately though: imminent hardware failure! But thankfully that turned out not to be the case.