Apple says the GPU inside the A14 Bionic is 50% faster than the one in the A13.
Apple said no such thing. Apple claims a 30% GPU performance boost compared to the A12 generation. Furthermore, observed performance for the A14 in the iPhone and the iPad Air (4th Gen) looks different in some important respects. iPhone benchmark scores for multi-core/multi-threaded workloads and GPU workloads look more modest than those for the iPad Air. That might be explained by the power management logic in the A14 doing what it is meant to do - the improved passive cooling permitted by the larger tablet format/frame/skin probably also allows those elevated multi-threaded scores (at the cost of slightly elevated power consumption) without causing any run away thermal build up. Looking at the A14 GPU scores for the iPhone and the iPad Air, however, that are so sharply different, makes one wonder whether the GPU in the iPhone has been locked to a lower frequency.