How to Adjust WiFi Video Quality on Your iPhone in iOS 10
If you're watching videos over Wi-Fi on your iPad, iPhone, or iPod touch, iOS 10 lets you adjust the quality for when you want to save data or need help watching a movie or TV show on a poor connection.
To do so, open up the Settings app and scroll down to Videos, then select the Wi-Fi option and choose between Good and Best Available. It's that simple.
This option is helpful for a few reasons. If you have a data cap for your home network, if your internet is slow and you want videos to load faster, or if you're using a business's Wi-Fi (e.g., Starbucks) and it's too slow from all the users hogging all that bandwidth.