Ars Technica has done some field tests using an iPhone 4s running iOS 8. Although the operating system runs decently, there are definitely some drawbacks that might stop iPhone 4s users from hitting update right away.
As shown in the chart, the OS performs more slowly across the board when opening apps. Safari took almost twice as long to launch, when compared with iOS 7.1.2. Camera took 1.8 seconds on iOS 8 versus 1.5 on 7.1.2. Just upgrading adds an extra 3 seconds onto boot time, although that is done so infrequently on an iPhone it probably doesn’t matter. In addition to slower app loading, it’s important to note that iPhone 4s users can’t benefit from every single feature.
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Like with iOS 7, AirDrop is unavailable. However, as Handoff relies on the same Bluetooth/WiFi communication mechanism that AirDrop uses, that is also not usable by iPhone 4s owners. The modern graphics frameworks introduced at WWDC, OpenGL ES 3.0 and Metal, are also not available as the GPU is too old. Ars also points out that enhancements like QuickType simply take up too much room on the cramped 3.5 inch display of the 4s. Although it ‘works’ on the device, there are definitely some areas of the operating system where Apple is clearly designing for the larger displays of the iPhone 5s and iPhone 6.
As with all new software upgrades, some performance hit should be expected. New features are more computationally intensive. That’s an unavoidable truth.
If you are happy with the reliability and performance of your iPhone 4s as it stands today, you might want to wait out iOS 8 for the time being. Usually, Apple follows with bug-fix updates that drastically help performance on older hardware. On the other hand, upgrading to iOS 8 still does unlock a wide variety of features to you (for free, let’s not forget) and the performance slowdowns aren’t that severe. It’s just important to be aware of the tradeoffs before you rush to hit that Software Update button.