Some iOS users are claiming that iOS 9’s Wi-Fi Assist feature — which uses cellular data to boost browsing speed when an iPhone is connected to a poor quality wireless network — has resulted in large amounts of unforeseen data usage.
The feature is automatically turned on when users update to iOS 9, and while the specifics of exactly how much data the feature uses aren’t entirely clear, users without unlimited data plans may want to turn Wi-Fi Assist off to avoid overages.
The off switch is somewhat buried, located in Settings>Cellular near the very bottom of the screen, under the cellular settings for all of the apps installed on the phone.
