IPhone 6 uses A LOT of bandwidth, no clue why?

IPhone 6 uses A LOT of bandwidth, no clue why?

Added (1). When my sister's iPhone is connected to the internet, it runs extremely slow. We have no clue why. Maybe an app? Anyone?

There's now a feature to set apps to auto update be sure it's turned off. There's also an option below to say whether or not you want to use cellular data for updates

The Wi-Fi signal strength also affects speed of browsing