2 Restore & Reset
If your iPhone is still sluggish after the previously listed steps, then you may want to perform a factory reset. Most of your content and data will remain, but you’ll need to update the various preferences and settings on your device. To do so, just go to Settings > General > Reset and tap on Reset All Settings.
The next thing to try after that may be a full restore from backup. Note this will erase everything on your iPhone so proceed with caution! If you back up regularly (as you should be doing), this isn’t much of a problem. You can do so by plugging your device into a Mac or PC with iTunes, or going to Settings > General > Reset and tapping on Erase All Content and Settings.