iOS 26 brings Adaptive Power , a smart battery saving mode that helps extend battery life when your device's battery usage is unusual. And it also has a notification setting when Adaptive Power slows down your iPhone's performance if enabled. Here's how to turn on the notification when Adaptive Power slows down your iPhone.
How to enable Adaptive Power notification to slow down iPhone
Open Settings on your iPhone, then tap Battery and select Power Mode to access battery management options on your phone.
Switching to the new interface, users just need to turn on Adaptive Power and Adaptive Power Notifications .

The result when Adaptive Power reduces performance to save battery will display a notification on the lock screen as below.

Adaptive Power mode makes smaller system-wide adjustments like dimming the display, slowing down background activity, and automatically switching to Low Power Mode when the battery is at 20%. Support for the feature is still limited to devices with Apple Intelligence, which currently includes the iPhone 15 Pro and Pro Max, along with the entire iPhone 16 lineup.