If you are not on an unlimited cellular or broadband connection, using a slow connection, or just want to limit the amount of data that your device consumes, iOS has a hidden feature to help you do just that.
The feature, called Low Data Mode, and there's a separate setting for both cellular data and your separate Wi-Fi connections.
According to Apple, the way Low Data Mode works is dependent on the apps in question, and that you should expect the following:
Apps might stop using network data when you're not actively using them.
Background App Refresh is turned off.
The quality of streaming content might be reduced.
Automatic downloads and backups are turned off.
Services such as iCloud Photos pause updates.
It's worth noting that third-party apps such as Google Chrome or the YouTube app will not change how much data it downloads as a result of this setting, expect that background refresh will be disabled.
Apple's own apps also respond to being restricted to Low Data Mode as follows:
App Store: Video autoplay, automatic updates, and automatic downloads are turned off.
Music: Automatic downloads and high-quality streaming are turned off.
Podcasts: The frequency of feed updates is limited, and episodes are downloaded only on Wi-Fi.
News: Article prefetching is turned off.
iCloud: Updates are paused, and automatic backups and iCloud Photos updates are turned off.
FaceTime: Video bitrate is optimized for lower bandwidth.
If that feels like an OK compromise to you, here's how you activate the feature.
20-pin USB-C magnetic breakaway connector
Go to Settings > Wi-Fi and from the list find the relevant Wi-Fi connection, and then tap on the "i" to bring up the information screen about the connection. From there, flick the switch on Low Data Mode.
Remember that each Wi-Fi connection you use will have its own independent Low Data Mode setting, so setting it for one does not influence another.
Go to Settings > Cellular > Cellular Data Options and flip the switch on Low Data Mode.
If you have a dual-SIM iPhone, you have independent control over each data plan.
How I upgraded my USB-C power banks and chargers to make them easier and more convenient to use (in pictures)