In our look at the iPhone 6 and 6 Plus, we found those devices' PowerVR graphics processors to be very efficient in terms of performance versus peak specs. Their battery life in an intensive graphics workload was quite good, as well.
Part of the magic at work there is the tile-based deferred rendering scheme employed by PowerVR GPUs. This approach differs from traditional immediate-mode rendering, used on most Radeon and GeForce GPUs. Although it adds more complexity in the GPU's front-end, it can pay dividends in terms of energy use and overall architectural efficiency. For this reason, quite a few mobile-focused graphics processors now use some form of tiling, though not all of them use fully deferred rendering like the PowerVR lineup.
If you'd like to understand tilers better, you owe it to yourself to check out this write-up from Imagination Tech explaining how tiling works in the context of modern PowerVR GPUs. The post is written by Rys of Beyond3D fame, and his writing is accessible as usual. He stops short of explaining deferred rendering in depth, but he promises a follow-up post to continue to story.