From Julie Clover's "iPhone 13 Pro Offers Significantly Improved GPU Performance Compared to iPhone 12 Pro" posted Wednesday on MacRumors:
Just a day after the iPhone 13 models were announced by Apple, the first Geekbench score has already surfaced. Someone has benchmarked the GPU performance of the iPhone 13 Pro, and there's a significant performance increase compared to the prior-generation iPhone 12 Pro.
The iPhone 13 Pro, or iPhone14,2, earned a Metal score of 14216, a 55 percent increase compared to the 9123 Metal score earned by the iPhone 12 Pro...
Apple has called the A15 chip with the 5-core GPU in the Pro models the "world's fastest smartphone chip," and says that it delivers "up to 50% faster graphics performance than any other smartphone chip." Given the difference demonstrated in Metal scores, that appears to be accurate, with the A15 in the Pro models outperforming the A14.
My take: For whatever reason, Apple this year put its extra cores into GPU (for graphics and machine learning) rather than CPU (arithmetic, logic, i/o, etc.).