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@Ghostie It doesn't. Background tasks are unaffected, it just kills the frontend. So the next time you open the app it has to start everything from scratch instead of just resuming where you left off (full RAM uses about the same amount of power as empty RAM, but restarting makes opening the app slower and wastes some cycles) You spend a few minutes each day in the app switcher swiping away apps instead of just turning the screen off (the screen is the biggest power consumer)
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@Ghostie modern phone OSs are highly optimized to try and squeeze the most time out of the limited energy they have available, so they already kill any tasks that look expendable