WiFi Assist is a new feature in Apple's latest mobile operating system that will automatically switch your iPhone from WiFi to your cellular data if your WiFi connection is too weak to properly load webpages or emails.
Apple claims the new WiFi Assist feature in iOS 9 should be increasing data usage only by a "small percentage" for most people.
But that's contrary to several complaints from users that claim their data usage has skyrocketed ever since updating to iOS 9.
Tech Insider's Alex Heath experienced a dramatic increase in his data usage:
—Alex Heath (@alexeheath) September 28, 2015
Even if WiFi Assist is overly sensitive, Apple assures that WiFi assist is smart about when it activates the feature. It says your iPhone won't use cellular data for apps running in the background, nor will it activate for some third-party apps that stream audio or video, or download large attachments like an email app.
But some iPhone users claim that WiFi Assist isn't as "smart" as Apple says it is. For this Reddit user, Wi-Fi Assist apparently activated itself for a huge iCloud Photo sync when this user was setting up his new iPhone 6s Plus in a spotty Wi-Fi area: This user reported a similar experience on a MacRumors forum:
Both these users faced issues with iCloud while setting up their phones, as iCloud Photo syncing (obviously) uses iCloud, and so does a settings and data restore (if you backup to iCloud).
The WiFi Assist feature is turned on by default. Follow these steps if you want to turn it off.