Detect if Apple is throttling your iPhone - slowApple CPU Benchmark Test
Is your iPhone slow?
Make sure LOW POWER MODE is off.
The test works for iPhone 6S and up
older models will have different ranges for what is good and bad.
For iPhone 6 and 6+ only: GREEN is 7 - 8 seconds YELLOW is 8 - 9 seconds RED is over 9 seconds
Sorry, this test will only help you if you are currently using an iPhone.
Please come back on your iPhone.
WHY IS THIS HAPPENING
Basically, Apple added software so certain iPhone models wouldn't unexpectedly shut down when the battery gets too old. But, some people think Apple didn't notify consumers because they wanted to push people to upgrade sooner.
I recently learned that Apple has a feature telling you if your battery needs to be serviced since iOS 10.2.1; I don't know how effective it is but if it appears it will be under "Settings > Battery" at the very top. Here is a link to Apple's explanation of the feature
HOW TO FIX IT
Get a new battery. Guess what, Apple has a steep discount for the battery replacement program during 2018 because of this issue! Here are some helpful links:
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.