iOS 9's Low Power Mode feature was designed to reduce the iPhone's power consumption by turning off battery draining features like background app refresh, animated backgrounds, Mail push, Hey Siri and automatic downloads among others. Some benchmarks have shown that it also significantly reduces the CPU's performance. That said, you can still make and receive phone calls and texts, making it ideal for situations when you are dangerously low on battery but expecting and important call or text. Emails have to be manually fetched while in Low Power Mode.
Can I still make and receive calls and send texts while in Low Power Mode?