There’s apparently a good reason why the latest iPhones haven’t seen a significant improvement in graphics performance.
According to the sources of the site The Information (via macrumors) Apple had planned a significant advance in the graphics capabilities of the iPhone 14 Pro, but this new graphics processor (GPU) was withdrawn at the last moment due to technical problems, unheard of for the manufacturer.
iPhone 14 Pro, GPU Ray Tracing problem discovered at the last moment
This new GPU notably included Ray Tracing performance, but Apple engineers were reportedly overambitious, and the prototype used way too much power, impacting battery life and causing overheating. The defect was discovered quite late in the development cycle, and Apple had to -almost at the last moment- abandon the processor to use a model similar to that found in the chip of the previous iPhone 13.
This event is apparently unprecedented in the history of iPhones, and explains why the improvements in graphics performance of the iPhone 14 Pro are only marginal compared to the leaps of previous generations.