1. Download an app that tells you the speed of your iPhone's main chip.
The app I'm using here is the free Lirum Info Lite, which provides a lot of information about your iPhone's internal activity.
2. In the Lirum Info Lite app, check the speed of your iPhone's main chip speed.
From the Lirum app's home screen, tap the options button on the top left (three horizontal lines) > tap "This Device" > tap "CPU" > and check the "CPU Actual Clock" against the "CPU Maximum Clock."
If both numbers are the same, then your iPhone isn't being throttled.
Here are the chip speeds for the iPhone 6 and newer.
Apple rolled out its performance throttling measures for the iPhone 6 and newer models to prevent unexpected shutdowns. Here are the original "clock speeds" – the measure of speed for a processor chip – for the iPhone 6 onwards:
iPhone 6 and 6 Plus: 1.4GHz
iPhone 6S and 6S Plus: 1.84GHz
iPhone SE: 1.84GHz
iPhone 7 and 7 Plus: 2.34GHz
Note that your older iPhone may not be throttled if you're running an older version of iOS.
For the iPhone 6, 6s, and SE, Apple's performance throttling feature was introduced in iOS 10.2.1. If you have one of those iPhones but an older version of iOS, your performance won't be throttled.
For the iPhone 7 generation, Apple's throttling feature was introduced in iOS 11.2. If you have an iPhone 7 generation model with an earlier version of iOS, then your performance won't be throttled.
Even if your older iPhone isn't being throttled, it could still be worth getting a battery replacement by Apple.
If your older iPhone's main chip is still running at the speed it should, you'd still do well to take advantage of Apple's discounted $29 battery replacement program. The discount program will only last a year, at which point the cost of replacing an iPhone battery out of warranty will return to the original $79 price.
Chances are that the battery capacity in your older iPhone model has degraded over the months and years that you've owned it, and you can get improved battery life with the new $29 battery replacement offered by Apple.
The best way to check on your older iPhone's battery life is to get it checked at an Apple store.
Unfortunately, battery apps simply aren't reliable in reporting the health of your older iPhone's battery. They might give you a general idea that your battery has degraded since the day you bought it, but I've seen wildly varying percentage numbers regarding my iPhone 6S Plus' battery health.
The best way to find out the health of your iPhone's battery is to take it to an Apple store. Set up an appointment and an Apple Genius will run your iPhone through the company's series of tests.
Usually, Apple recommends that you get a battery replacement when your iPhone's battery can only retain 80% of its original capacity. With Apple's new discounted $29 battery replacement program in mind, however, I'd personally get it replaced even at 90% capacity.
You can get in touch with Apple Support remotely over the phone or even Twitter to find out whether you need a batter replacement or not, but they won't tell you specific details about your iPhone's battery health. The company's support teams will only tell you whether your battery is "fine" or not.
In its statement regarding the performance throttling controversy, Apple said it will also issue an iOS software update early this year "with new features that give users more visibility into the health of their iPhone’s battery, so they can see for themselves if its condition is affecting performance."
