The phone’s battery management IC monitors cell temperature through thermistors and, when things get warm, tells the charger to back off. It’s playing a graphics-heavy game while plugged in, which stacks SoC thermal output on top of charging heat. That’s why a 100W-rated phone charger can sometimes run cooler during a top-up than a 20W charger from 2019. Apple’s documentation on Optimized Battery Charging is explicit about this being the goal. The version of this advice I’d give someone who genuinely wants their phone battery to last four years instead of two isn’t “use a slower charger.”