Apple’s iOS has less to no lag simply because at a primary Os level, it doesn’t do much compared to Android. It’s a sandboxed atmosphere whose User interface is a grid of icons. Because it needs less resources, Apple is ready to leverage a number of the iPhone’s processor and GPU to make things appear softer.
However, Android does much more at the system level. Live wallpapers, icons, defaults and intents, etc. As a result of all that stuff, Android phones can’t use as many system resources to make things softer. After all, it wasn’t until presently did hardware get to a point where Google could implement things like Multiple buffering and VSync that Apple’s been using for many years. However, the hardware is getting there.
Looking at the jump from the Nexus 7 to the Nexus 4, Android hardware is quickly getting to the point where it’s finally able to match the feature set Android offers.