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Sometimes I notice that the Internet connection in my house becomes REALLY slow.
Disabling WiFi on my wife's iPhone fixes this.
But how can I tell which app/process does this?
iCloud backup is disabled. Dropbox is not installed. The "data usage" report shows cellular data, not wifi, I assume. What do I do? How do I debug this?
If your router doesn't have a way to log bandwidth usage, your best bet would be to run every suspect app on its own.
By that, I mean you should go into the App Switcher (double-press the Home button) and swipe upwards on all of the apps in that list. That kills all of the apps, so the only ones that'll be running are the ones you open afterwards. Kill all of the apps, then let one run and see if it has any effect on your wifi.
Not a great solution, I know, but to my knowledge, there isn't an easy way to figure out your problem.
You are correct Apple only reports data usage for cellular and not WiFi so you’d need to connect to a WiFi network you control and get the statistics from the router. I like using Ubiquiti Edge Routers that are in the $50 range if you are starting out on managing traffic on a network.
You also might just be able to use a service like OpenDNS to track the DNS lookups and figure out the names of the servers being contacted to track the data usage and guess what apps are running. The iOS 12 activity graphs are so much more detailed - hour by hour application usage and details if they are on screen or off screen will severely narrow down which apps are accessing the network. If an app is taking CPU or using energy in the background, you can be sure it’s to do network access.
