Your iPhone is getting a feature that saves battery life, but you'll have to make some compromises
Apple's SVP of Software Engineering Craig Federighi said during the announcement that the setting "pulls levers you didn't know you had" to give you up to three hours of added battery life with typical use.
That's great for iPhone users whose iPhones barely make it a day on a single charge.
Even without the low power mode, iOS 9 has been optimized to run more efficiently and squeeze an extra hour of battery life out of the iPhone.
However, the description below the on/off switch for Low Power mode in iOS 9 says "Low Power mode reduces performance and networking activity to extend battery life." That means the mode will shut down certain functions of your phone in order to save the battery.
If you've ever used an Android phone, you might have some idea how Low Power mode in iOS 9 might work. If you haven't, you could expect the following.
On many Android phones, power-saving features will turn off certain antennas like GPS or WiFi, and some will turn off automatic syncing for things like email, which means you'd only get new emails when you open your email app. You can pull these network-related "levers" yourself in iOS's settings, but it's nice to have them all pulled with a single flick of a switch rather than digging through Settings.
However, the "levers" Federighi might have referred to that we didn't know we had could be the internal hardware in your iPhone.
On some Android phones like the Galaxy S6, for example, the power-saving feature will actually slow down the internal hardware, which slows down the overall performance of the phone. You can still go about your typical usage, but it's not as quick or smooth.
While we don't exactly know yet how much of an impact on performance the low power mode in iOS 9 will have, it's likely your iPhone would slow down a little when you turn on the low power mode, which might be better suited for limited usage rather than typical usage.