An upcoming incremental update to iOS could result in some iPhone models throttling the CPU, in certain situations. The feature (not a bug) is basically a workaround for a degraded battery that might otherwise turn off an iPhone unexpectedly. This feature already exists older iPhone models, and soon will be rolled out to newer models.
"For iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone SE, iPhone 7, and iPhone 7 Plus, iOS dynamically manages performance peaks to prevent the device from unexpectedly shutting down so that the iPhone can still be used. This performance management feature is specific to iPhone and does not apply to any other Apple products. Starting with iOS 12.1, iPhone 8, iPhone 8 Plus, and iPhone X include this feature; iPhone XS, iPhone XS Max, and iPhone XR include this feature starting with iOS 13.1," Apple states in a support document.
The iOS 13.1 update follows the iOS 13 roll out that coincided with the launch of the company's iPhone 11 series. Overall, iOS 13 is a nifty update—it introduces things like a system wide dark mode and better handling of photos and text messages, among a plethora of other tweaks.
In a somewhat unusual move, Apple decided to reserve some planned features for a slightly later date, with iOS 13.1 slated to arrive on September 24. It will introduce things like the ability to share an ETA with contacts from Maps, Siri Shortcut actions, and a few other things. Apparently CPU throttling will be thrown into the mix as well.
Apple caught a lot of heat in 2017 for throttling performance on some iPhone models without initially divulging that practice. It would later admit to doing it, and as a mea culpa, it temporarily dropped the price of its iPhone battery replacement program to $29.
What about the new iPhone 11 series? The new handsets apparently have hardware and software features designed to trade performance for battery life, when necessary. However, Apple claims the iPhone 11 models do a better job of this than any of its previous iPhone handsets.
"iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max power needs are dynamically monitored, and performance is managed so that it can address these needs in real time. The system is more advanced than previous iPhone battery and power-management systems and allows your iPhone to reduce performance impacts from battery aging," Apple says.
This can manifest in different ways, according to Apple, including longer app launch times, lower frame rates while scrolling, backlight dimming (this can be overridden in Control Center), lower speaker volume, gradual frame rate reductions in some apps, disabling the camera flash (during the most "extreme cases"), and having to reload apps that refreshed in the background.
According to Apple, the iPhone 11 Pro Max lasts up to 5 hours longer than the iPhone XS Max, and the iPhone 11 Pro lasts up to 4 hours longer than the iPhone XS. The iPhone 11, meanwhile, lasts up to 1 hour longer than the iPhone XR, Apple says.