iOS 9 is also optimized for better energy and storage conservation. According to Apple, iOS 9 users can gain up to an hour more of battery life on a full charge via facedown detection (which leaves the screen off when a notification is received and the phone is lying on its display), optimized backlight algorithms, adaptive sleep delays and a more efficient low-power idle state.
To manually conserve energy, there is a new preference under Settings called Battery that has the option to enable Low Power Mode. This new setting scales back some features such as background app data refresh; throttles the GPU and CPU from entering the most power-hungry processing states; and disables animations and some of the more taxing visual effects.
Despite these newfound efficiencies, the device didn't feel slow when I used it in Low Power Mode (the battery indicator turns yellow when the mode is active). You can turn on the Low Power Mode manually or have it automatically enabled when the iPhone pops up a low battery warning at the 20% or 10% charge marks. Once the phone is charging, it will automatically switch back to normal operations when the battery is full or near full.
There are several ways Apple has worked to improve storage, including the use an optimization technique called App Thinning.
App Thinning has three components: App Slicing, On-Demand Resources and Bitcode.
App Slicing: There's a reason a single app can run on an iPhone, an iPad and an iPhone touch: Each app can contain different sets of code (such as images scaled for Retina and non-Retina displays) that execute depending on the device. Even if that code is not needed by the individual device, it still resides in the app, taking up storage. With App Slicing, the App Store only delivers what's required for each device. That means no more unnecessary code being bundled and downloaded, saving space and reducing download size.
One note on App Slicing: iOS 9 allows developers to create 64-bit-only apps. Since iOS 9 runs on devices that are not 64-bit capable (such as the fifth-generation iPod Touch; the iPhone 4S, 5 and 5C; the first-generation iPad mini; and iPads 2 through 4), those 64-bit-only apps won't even display in the App Store on unsupported devices.
On-Demand Resources: In iOS 9, the App Store can retain separate app resources so that some segments are only delivered as needed. Think of a video game: When a player is on Level 1, the game doesn't require any other level data except what's being used; when Level 2 is required, it's downloaded, and Level 1 is deleted. iOS reclaims that space on the fly.
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