I don't know what it is, but OS X has a hard time with PDF's. My new iPhone 6s+ scrolls through PDF's perfectly smoothly, while my MBP (mid-2104) stutters when scrolling through a large PDF. Is it the A9 or is it the iOS that makes it shine in the PDF department? Maybe OS X is not optimised correctly? It's frustrating to see that an i7 cannot smoothly scroll through text and pictures.