Developer Matt Birchler ran a 20-run benchmark using his app Quick Subtitles on the iPhone 18 Pro, iPhone 17 Pro, and an M4 Pro MacBook Pro. The 18 Pro was about 50 percent faster than the 17 Pro overall, but it warmed up more quickly and throttled harder over the course of the runs. Birchler reported that the 18 Pro spent roughly half of the runs in a "serious" thermal state without external cooling.
The 18 Pro's improved vapor chamber—three times the surface area of the previous generation—is doing its job by pulling more heat away from the processor and spreading it across the device. That makes the phone hotter to the touch, but it's an expected trade-off. Even so, the larger chamber wasn't enough to prevent throttling during sustained heavy work.
To test the limits, Birchler placed a cold can of La Croix on the back of the phone and reran the benchmark. The first run hit 429.9 words per second, and the 20th run still managed 400.4 words per second—compared with a drop to 320.9 without the can. With the can, the phone never rose above a "fair" thermal state, and sustained performance was about 14 percent higher.
The experiment is a practical demonstration of passive cooling limits in modern smartphones. It also shows that even a flagship with a larger vapor chamber can benefit from external heat sinking, however unconventional the method may look.