Wrong conclusion... the application waits on a vertical sync after each frame before it continues. If it's just a little bit too slow, it will miss the vsync from this frame and wait for almost a whole new frame to be drawn before continuing. Given the fact the application sometimes seems to be just fast enough for a redraw within one frame time, chances are the actual performance degrade is just a tad below the original one. It certainly is not as dramatic as the movie leads you to believe.