When Apple announced iOS 9 at its recent Worldwide Developers Conference (WWDC), the iPhone maker touted improved efficiency across the operating system, apps and key technologies as well as a new “Low Power” mode that would extend an iPhone’s battery life.

Low Power mode dials back processor performance to extend an iPhone’s battery life and a benchmark test published Thursday reveals exactly how much performance is cut when this battery saving tactic is employed.

Using popular benchmarking tool Geekbench 3, MacRumors ran a benchmark test on an iPhone 6 Plus running an iOS 9 beta.

The benchmark test showed that in Low Power mode the iPhone 6 Plus’ multi-core score dropped from 2,891 to 1,751, and single-core performance dropped from 1,606 to 1,019. The publication reported similar results on an iPhone 5s running an iOS 9 beta in Low Power mode, which saw a 40 percent drop in performance.

Low Power mode kicks in once an iPhone’s battery charge drops to 20 percent and then 10 percent. At each increment, the user is presented with a popup prompting them to toggle Low Power mode on or off. Users can also turn Low Power mode on in the new Battery section of the Settings app.

Along with dialing back processor performance, Low Power mode also disables battery-hungry tasks such as Mail Fetch, Background App Refresh, motion effects, and animated wallpapers, notes the report.

At WWDC, Apple claimed Low Power mode in iOS 9 could add as much as three hours to an iPhone’s battery life.

Apple also promises an extra hour of battery life with no user action required at all. More efficient core software and less resource-hungry apps are combined with new technologies in iOS 9 to extend an iPhone’s battery life. For example, ambient light and proximity sensors allow iOS 9 to know when an iPhone is face down on a table and not being used, allowing it to stop the screen from turning on unnecessarily.

iOS 9 is currently in its second developer beta, and these performance numbers are likely to improve further in upcoming beta releases as Apple tweaks and enhances its new operating system.

iOS 9 is due for public beta release in July followed by its general release sometime in September.

Image credit: Kārlis Dambrāns | Flickr | CC BY 2.0