Remember all of that drama when Apple admitted to throttling iPhones in order to save battery life? Well, this tool is a response to that. Not only will it tell you if your phone is being slowed down, but you can also disable any throttling or use it to determine if you need your battery replaced.
Install iOS 11.3 in Settings > General > Software Update on your iPhone.
With iOS 11.3 installed on your iPhone, there's a new section on the Battery Settings page that details your battery's health. View it by going to Settings > Battery > Battery Health (Beta). There you will see your battery's current health, as well as a description of how its health impacts performance.
If your iPhone's performance is being throttled, you will see a statement similar to the one above, where you can disable the feature if you want. However, if your iPhone does randomly shut down due to battery health and performance issues, this feature will automatically enable itself once again to prevent future shutdowns.
Also worth pointing out is that if iOS is unable to determine the health level of your battery, or if the level is low enough, you will be directed to take your iPhone to an Apple Store or an authorized service center to have it tested and possibly replaced.
Apple - USE TAG
reading•iOS 11.3: How to check if your iPhone is being slowed down
Nov 15•Black Friday 2018 iPhone deals: $150 off iPhone XR and XS, $400 iPhone X gift card
Nov 15•Costco Black Friday 2018 deals: $300 Dyson, $800 Surface Pro bundle and more
Nov 15•Sick of Facebook stalking you? This startup wants to reinvent personalization
Nov 15•The iPhone XR is now available unlocked from Apple