ios 9 five best features
Above, Apple's Craig Federighi introduces iOS 9, which brings a raft of new features including ad blocking, updated search and deeper Siri intergration. Many users experienced download problems on the software's launch day. Justin Sullivan/Getty Images

Apple announced on Monday that, based on data from the App Store Saturday, iOS 9 had the fastest adoption of an iOS update ever. Over half of iOS devices are now using the latest update. "iOS 9 is off to an amazing start, on pace to be downloaded by more users than any other software release in Apple’s history,” said Phil Schiller, SVP of marketing at Apple.

The news is a great feat for a company scarred by poor publicity surrounding its previous mobile device update. Users complained that iOS 8 took up a lot of space and caused issues on devices. iOS 9 has focused on speed and stability, improving the core of the system rather than introducing a long list of new features. The update does, however, include some big changes, notably the addition of better Spotlight searching and app suggestions, a newly-designed font, split-screen multitasking for iPads and a new app switcher.

Two major factors likely to have contributed to iOS 9's success are its smaller file size and wide range of device compatibility. iOS 9 only requires 1.3GB of free space to complete the update, compared to 5.8GB for iOS 8. iOS 9 also runs on the same devices as iOS 8, a change of pace for a company that usually drops support for its oldest device. The iPhone 4S, released in 2011, is the oldest iPhone that can run iOS 9.