With iOS 9 came a load of new settings to wrap your head around. Some are great, while others are not so good, especially for those who have to keep a close eye on their data consumption.
If you go to Settings, then Mobile Data, and scroll right to the bottom, you’ll find Wi-Fi Assist. It’s been turned on by default once you update to iOS 9. What this new setting option does is to automatically switch you over to a data connection once it detects a poor Wi-Fi connection.
To be fair, Apple probably introduced this setting with seamless connectivity in mind. But for those of us with a 2GB ration of data each month, we’d rather stick with seeking out a Wi-Fi connection than have our Silicon Valley stream uninterrupted. Instant gratification just won't justify a sky-high phone bill.