Peacekeeper just so happens to test many of these same tasks. Looking at the iPhone 5s, the benefits from the new FTL JIT are highlighted by a 34% jump in performance with iOS 8. Comparing the iPhone 6 to the 5s with iOS 8 shows a 13% improvement thanks to the A8 SoC.
Principled Technologies' WebXPRT 2013 is an HTML5-based benchmark that simulates common productivity tasks that are traditionally handled by locally installed applications, including photo editing, financial charting, and offline note-taking.
The iPhone 5s sees minimal improvement when running the new version of Safari in iOS 8 in WebXPRT 2013. The 6 Plus sees about a 15% improvement over the 5s, similar to the gain in Peacekeeper.
For some reason the iPhone 6 scores show a lot more variability (scores ranged from 425 to 554) than all of the other phones in this chart. After running the test several times, I threw out the lowest score and averaged the rest to arrive at the value shown in the chart. I’m not sure what’s causing this, but it’s not thermal throttling. It still feels cold after completing the benchmark and I didn’t notice this behavior with any of the other benchmarks we ran.