Okay so interesting you should ask my moms been texting me all day telling me she's at her phone carrier getting an iPhone 6 and she got a brand new 5s two weeks ago . The sales rep told her the 5s is having so many problems with the update . read more
On the iPhone 4S and iPhone 5, the boot time was increased by several seconds with iOS 9, while it is nearly the same on the iPhone 5S. Camera performance across all three devices is slightly slower with iOS 9/iOS 9.0.1, although there’s a very small difference with the iPhone 5S. read more
It should not slow down the device. It may slow it down at the start, because this is a new OS and it still needs to be optimized quite a bit. However, iOS 9 has support for legacy devices so within the next few weeks it should speed up your device and give you extra battery life. read more
Based on every other version of iOS on older iPhones, there's a high possible chance that the next major version can slow the iPhone 5 down. read more