Actually I don't own an iPhone X :P Our core user base tends to use 6 and above (approx 2 generations behind) and we dogfood regularly with a huge group of beta testers (thousands) with a huge range of hardware. We try our best to make sure the average use case is considered.

Considering the average is good but it still pushes the performance treadmill forward.

If we had some magical way to make the dev treadmill run slower than the user treadmill, we'd see better architectures in many cases. In that alternate world it would be easy to precompile the javascript and load 20 times faster.

As an iOS dev, I use my personal phone (iPhone X) for the bulk of my dev/test cycle but also keep around an old-ish iPod Touch running the previous version of iOS that I dev/test on periodically as well. If something doesn’t perform adequately on the Touch, I tweak on it until it does.

iOS users tend to run newer devices generally which might lead some devs to feel that this extra optimization isn’t worth the time it takes, but I disagree because it doesn’t benefit only users with older devices – it’ll make your app run better on newer devices too and make it that much more unlikely that your app gets killed or causes the OS to kill other apps in multitasking scenarios (iOS kills resource hogs much more readily). In short, everybody benefits.

With this same thought process, I plan to pick up one of the 120hz iPad Pro models to test on soon. It’s one of the most powerful iOS devices available, but driving 120FPS isn’t easy and given how many apps fail to hit the target of 60FPS on similar hardware I’d bet that a huge number, perhaps even most, don’t come anywhere close to running at 120FPS even with the extra horsepower.

That creates opportunities for other developers to build a better product.

That kind of optimization is happening much more on Android than on iOS, the install base of low end devices is much, much larger.