Android screens are rendered widget by widget, pixel by pixel for every screen modification (scroll, zoom, item state changed, etc.) and that means a lot of work is being done for every frame. This is a legacy of the original spec not requiring a dedicated gpu. Modern devices are getting them but the acceleration is sort of hacked into that gpuless model. iOS on the other hand didn't start with such a limiting assumption and paints everything to an open gl surface with a fixed camera. Most screen modifications like scroll and zoom are just basic gl transformations that don't require a re-rendering event. That's why it looks so smooth.