Example - my 13 ProMax or iPad Pro 11 is working great.
Then something stops working correctly. Mail not updating timely, Messages that vanish off screen after send, apps that start running slow, Safari tab refresh that crawls, etc ..... It generally happens in groups - not just a specific app only.
When I try a relaunch of the app, or a reboot of the device, sometimes it helps. Check of Apple Services show all green. Doesn't matter it it is WiFi, 5G, LTE. Check of up/down speeds and other shows all is well. Then suddenly it all starts running fine.
If I have an app that runs fine in airplane mode I never see this. It has been happening more often in 15 as it matured. Too soon to say about 16 but am seeing some of the same. It is almost like the device has to do an extracurricular check (phone home) before running. Didn't see this with 14.
Clear as mud I'm sure.