Because the iPhone 5 and 5C models are on the bottom rung of the iOS 10 compatibility ladder, if any iPhone is going to experience issues running the new iOS, these two models are the likely culprits. While the new iPhone 7 models feature Apple's latest A10 processor, the iPhone 5 and 5C are four generations behind with the A6 chip. (In between sit the A9-based iPhone 6S and SE, the A8-based iPhone 6, and the A7-based iPhone 5S.)
In addition, the iPhone 5 and 5C feature only 1GB of RAM; the iPhone 6S and onward incorporate 2GB of RAM.Until iOS 10 is released on Tuesday, we can't put it through its paces in CNET Labs testing, but it's reasonable to speculate that the older A6-based iPhone 5 and 5C might struggle with powering all of iOS 10's bells and whistles.
CNET's Sharon Profis has used the Gold Master version of the iOS 10 beta on an iPhone 6 and feels that it slowed down performance considerably. Of course, the official release of iOS 10 will offer enhancements to the Gold Master beta, so perhaps all compatible devices will run iOS 10 swimmingly.
Then again, maybe not. We aren't advising against updating to iOS 10, but would urge iPhone 5 and 5C owners to proceed with caution. Let the official release of iOS 10 breathe for a few days to see if there are any reports of it causing performance issues before taking the plunge yourself.
A ray of hope
A day after iOS 10 launched, Ars Technica called iOS 10 "a pleasant surprise for the iPhone 5 and 5C" and recommends the update, something it hasn't done for the oldest supported iPhone and a new iOS since the iPhone 3GS and iOS 6. Ars found that the iPhone 5 and 5C felt roughly the same on iOS 10 as iOS 9 but identified three areas where you might be left wanting: games, multitasking and battery life.
More than performance, the smaller screen of the iPhone 5 and 5C might be the bigger issue when moving to iOS 10. In addition to complaining about the lack of swipe to unlock and needing to tap Home twice to unlock either iPhone model (a workaroundexists for Touch ID-enabled iPhones), a few Reddit users say that the updated Messages app feels cramped on the 4-inch iPhone 5 or 5C display, but the general consensus seems to be that you won't take a performance hit.