1. Their PowerVR GPU is faster than the one on N900 2. Their screen runs at half N900's resolution 3. Their OS is somewhat optimized for singletasking with plenty of resources spared for UI (with some background-enabled apps) 4. I bet 'make the UI as fluid as possible' is one of Steve Jobs' prime directive to his team. So this has always been optimized and yet optimized more. It's always been like that since v1.0.