I recently learned that Apple has added a full suite of battery features allowing you to see usage history, maximum capacity, battery performance and you can even MANUALLY DISABLE THROTTLING. It is available with iOS 11.3+, the battery features are much more limited before this update. You can find this feature by going to "SETTINGS > BATTERY" Here is a link to Apple's explanation of the features

HOW THE TEST WORKS 

It works like most benchmarks. The test is written in javascript which runs on the iPhones browser. It just simulates work by calling a javascript function and then it calls the function hundreds of thousands of times while keeping track of how long it takes the iPhone to complete the task. Then it compares the score to other peoples self-reported scores, iPhone model, battery health and current iOS version.

The test only shows if you are being slowed down when the test was run. Because a combination of the devices temperature, battery charge and the battery’s impedance (affected by health) triggers the throttling, it is unlikely anyone will be throttled all the time. Devices that are throttled will get fairly inconsistent scores with my tool precisely because the CPU speed is fairly inconsistent. If you want additional verification you can look up your battery's health. There are free apps on the app store which try to guess at your battery health which are probably accurate enough. Here is a good one to get you started.. You can also go into an Apple store and have them run battery diagnostics on your device. I've heard rumors you can get access to some of these diagnostics using the Apple Support App available on the App Store but I haven't tried it myself.

If you aren't convinced yet that my test can detect throttling when the test was run, then turn on the Low Power Mode feature which throttles an iPhones CPU speed to conserve battery and run the test again. By slowing down the CPU it will take longer to perform the test and because there is no way for a website or javascript to know if Low Power Mode is enabled you know those numbers above aren't being faked. Good luck and thanks for visiting!