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Turn off 'parallax' motion user interface
If you slowly jiggle your iPhone in your hand, you may notice the "parallax" effect of three-dimensional icons sitting on top of your wallpaper. This effect uses internal hardware to sense the direction, speed, and motion of your movements. This, as you might expect, will drain the battery. Though flashy and visually quite cool, it's not wholly necessary if you're a power user.
To disable the "moving wallpaper" feature, head to Settings > General > Accessibility > Reduce Motion, then flick the switch in the on position.
Disable Siri's 'raise to speak' feature
Siri can be activated by holding your iPhone close to your face, as though you're having a face-to-face talk with the intelligent assistant. But the proximity hardware sensor is always checking to see if you are nearby. This may impact the battery over the course of the day.
Head to Settings > General > Siri, then disable the Raise to Speak button to disable the feature. The alternative isn't so bad: You can still hold down the home button to speak to Siri.
Prevent Spotlight Search from indexing unnecessary files
Spotlight needs to index your entire phone's contents regularly so that it can serve up the best results. This may not use up a whole lot of processing power, but it's certainly helpful in the long run by removing items from the list that you don't use.
You can select which areas of your phone to index by heading to Settings > General > Spotlight Search and tapping any item you wish to exclude from the search index.