It was true all along: Apple really throttles iPhones. It's not to force users into upgrading, though, but to avoid random shutdown issues caused by degrading batteries.
Sure, it can be considered a good way to prolong an iPhone's lifespan, but on the other hand, the Cupertino brand shouldn't have kept it more or less a secret. If users knew that their iPhones were slowing down because of aging batteries, then a lot of them could have gotten the cells replaced instead of buying a new phone, an option that's a whole deal more affordable.
Now if you've noticed your iPhone lagging behind in terms of performance but can't be too sure, there are some ways to check for it.
Use An Identification Tool
An app that can give you a closer look at the inner workings of your iPhone is just the thing. One good example of that is Lirum Device Info Lite, not to mention that it's free.
Assuming that you installed the tool, all you have to do here is fire it up, tap the drawer button in the upper-left corner, and hit "This Device" and then "CPU." There, you should see "CPU Actual Clock" and "CPU Maximum Clock," and if the speeds match on both counts, then you should be good to go.
For reference, the clock speed of the iPhone 6 and 6 Plus should be at 1.4 GHz, iPhone 6s and 6s Plus at 1.84 GHz, iPhone 7 and 7 Plus at 2.34 GHz, and iPhone SE at 1.84 GHz.
Use A Battery Checker
Another way to go at it is to install an app that can check your battery's status. Of course, there are a ton of apps in the App Store that can do this, but just to get things started, you can go with Battery Life.
It's simple to use too. You just have to open it up, and it should tell you the "wear level" of your iPhone's battery.
Needless to say, your iPhone likely isn't being throttled if it doesn't find any kinks.
Go To An Apple Store
Arguably the best way to see whether your iPhone's battery is still fine or not is to have it checked by an Apple Genius. That's because apps can only do so much and yield varying results, though they can still give you an idea of how your battery is doing.
How To Fix
If you've determined that your iPhone is being throttled, you can either take advantage of Apple's battery replacement program or do it yourself, which is where iFixit's kit comes in.
Until December 2018, the Cupertino brand is slashing down out-of-warranty battery replacements from $79 to $29. As a side note, this offer is worth taking up even if your battery isn't worn out yet since it's a time-limited but huge discount, after all.
Alternatively, you can check out iFixit's battery replacement kit. It's complete with the instructions and tools you need to replace the battery yourself. It also costs $29, though, but one reason to go for it is because it also has kits available for the older models that Apple's program doesn't support such as the iPhone 4s, iPhone 5, iPhone 5s, and iPhone 5c.
Things To Take Note
Before wrapping things up, you should know that your iPhone may not have been slowed down despite a bad battery under the hood if it's running an older version of iOS.
iPhone 6 and 6s and iPhone SE units running on any version earlier than iOS 10.2.1 aren't being throttled. The same goes for the iPhone 7 and iPhone 7 Plus running on any version earlier than iOS 11.2.
It should also be noted that Apple promised to release new features that'll let users check on their iPhone batteries' health, and when those roll out, that means you no longer have to rely on third-party apps to determine whether or not you need to replace your iPhone's battery.