Faster performance, better battery life
As in El Capitan, which beta users have said runs faster and smoother than its predecessor, performance in iOS 9 gets a boost across the board. Apple's built-in apps can already take advantage of Metal, which better utilizes the CPU and GPU, translating to faster app launches, smoother animations and improved scrolling. Metal is a new set of APIs designed to replace OpenGL as the standard programming language for Apple's devices; it's used for desktops and laptops as well as iDevices. (Third party apps need to be rewritten to take advantage of these features.)
There are also smarter behaviors to improve battery life: the built-in sensors determine whether the iPhone display should remain off when alerts are received (such as when it's in your pocket or lying face-down on a table). And there's a low power mode that can be activated in Settings>Battery or by toggling on the lower-power mode when the iPhone warns that battery life is low.
iPad users get new multitasking features. While apps have been able to multitask for a while now, the iPad interface only handled one app on the screen at a time -- a limitation or a feature depending on your perspective. This changes with iOS 9 with the addition of Split View, Picture in Picture, and Slide Over, each of which lets the user interact with two apps at once. Some might say that Apple is catching up to rivals' features with a few of these additions, but they're welcome options nonetheless.
Little tweaks (and apps) that add up
Another welcome addition is that devices with Touch ID now require six-digit passcodes for better security. The more digits there are, the harder it is for unauthorized users to guess at passcodes. I would have required six-digits for everything, perhaps even a passphrase, but this is certainly better than four-digit passcodes. Baby steps.
Several apps get noteworthy updates, but the biggest change comes to Notes, which receives many of the features found in El Capitan. As in OS X, Notes can understand dropped-in content from other apps, including the use of a sharing pane that allows you to add to Notes from within other apps. You can also add photos, turn lists into checklists, and use your fingers to sketch out ideas. Notes isn't just for text anymore.
There are also two new apps: the one called News allows you to browse news via custom sources that you choose, while the Android-to-iOS app will help Android users migrate their data over to the iPhone. Apple CEO Tim Cook is constantly referring to switchers to the Apple mobile platform, and now those users can easily migrate existing data from one platform to the iPhone.
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