Software developer Matt Birchler tested an iPhone 18 Pro, an iPhone 17 Pro, and an M4 Pro MacBook Pro with a benchmark from Quick Subtitles, an audio transcription app he built. Each device ran the test 20 times with the same built-in benchmark file. Apple’s on-device transcription model performed the work.

The iPhone 18 Pro delivered between 447.9 and 320.9 words per second across the 20 runs. Birchler measured a 50 percent gain over the iPhone 17 Pro. Both phones heated up. The 18 Pro warmed more slowly at first, then became hotter more quickly and felt warmer in the hand.

The phone’s vapor chamber has three times the surface area of the version used in the 17 Pro series. This sealed component contains water and a metal wicking structure. Heat makes the water evaporate near the processor, and the vapor condenses in cooler parts before the wick returns the liquid to the hot area. Birchler attributes part of the 18 Pro’s higher performance to this larger chamber, alongside its more capable chip.

During the regular 20-run test, the 17 Pro may never have matched the 18 Pro’s performance, but it experienced less throttling. The larger chamber pulls more heat from the processor and distributes it across a wider area, which also makes the phone feel warmer. Birchler said the design reduces performance loss under heat, and passive cooling still causes some throttling.

For a second 20-run test, Birchler placed a cold can of La Croix against the back of the iPhone 18 Pro. The first run reached 429.9 words per second, and the twentieth still reached 400.4. Without the can, the phone spent about half its runs in iOS’s serious thermal state. With it, the highest state was fair. iOS reports the levels nominal, fair, serious, and critical to developers.

Birchler estimated that the cooling trick could make heavy work 14 percent faster if similar results appear in other workloads. The experiment used La Croix, but its cooling effect came from the chilled aluminum can.