Many people force quit all apps multiple times a day, in the assumption that this will safe their battery.
When you force quit an app, you take it out of your phone’s temporary memory, known as RAM, and send it to a terminated state. The next time you open that app, your phone has to load it back into RAM all over again, which takes up more CPU than if you were to relaunch the app from its “standby” state.
Not only does this high CPU usage drain your phone’s battery, you’ll also have to wait longer for your phone to switch between apps and load apps that you’ve force quit.
It is better to leave the apps in standby mode, only when an app gets unresponsive or stuck you should close it. See support article: https://support.apple.com/en-us/HT201330 and an email response from the Apple management: https://www.macrumors.com/2016/03/10/force-quitting-apps-doesnt-help-battery/
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