For the UI icons, do we actually "render" them real-time, or they are just images that are stored in the memory, and we just copy paste the pixels (maybe also for the background)? I feel like it's not worth the time to render them than just save the pixel information since they are not complicated nor changing over time. (But I guess the clock app icon for iPhone is still rendered right?)
Unity is doing some cool things with mobile games. Check it out: https://unity.com/solutions/mobile
Lots of modern mobile UIs have slick animations and such, which actually probably require compositing on the part of the OS. Windows 7 used to do something like disabling compositing and falling back to flat graphics on power saving mode. Why don't phones do this?