The iPhone SE iOS 12.3.1 update is a small maintenance update, but the latest version of iOS 12 is causing problems for some iPhone SE owners. If you’re thinking about moving your iPhone SE to iOS 12.3.1, make sure you’re fully prepared for the move.

iOS 12.3.1 is a bug fix update and it could have a noticeable impact on your device’s performance. Most iPhone SE owners should upgrade to iOS 12.3.1, but some might want to take a rain check.
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https://www.gottabemobile.com/7-things-to-know-about-the-iphone-se-ios-12-update/