Reports have surfaced the last few days of iPhone users claiming to see heavy mobile data drain after updating to iOS 9. The suspected culprit? “WiFi Assist.” Wi-Fi Assist is a new feature that is enabled by default in iOS 9 which switches to cellular data when Wi-Fi signal is week, to ensure that you always have a solid connection.
It’s still unconfirmed if the new feature is directly responsible for the extra usage, but based on some tests I preformed, I lost a considerable amount of data while connected to WiFi at work compared to previous months when my iPhone was still on iOS 8. The stealth switch to cellular network when you think you’re using Wi-Fi could mean you end up using more cellular data than you planned.
Conclusion: If you’re on a limited data plan, switch it off, better safe than sorry.
To disable the feature, go to Settings->Cellular->scroll down to Wi-Fi Assist, and turn it off.
