Apple iOS9 Wi-Fi Assist

iOS 9 now will "help you out" by having the device switch to cellular data if it thinks your Wi-Fi connection is too slow. This could end up using more of your cellular data than you'd like. This appears to be turned on by default after the upgrade. You can turn it off by going to Settings -> Cellular then finding Wi-Fi Assist at the bottom of the screen.

General WiFi iOS9 Apple