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My iPhone 5 went from iOS 6 to 7, 8, and 9. The worst issues I had were with 6, which had such terrible wifi I actually exchanged a few phones before giving up eventually and then finding iOS 7 sorted the problem. Each update brought benefits and I can honestly say it was running as well or better on 9 than it had at any point previously.

Running really well on iOS 9 here too. iOS 6 had really slow WiFi, iOS 7 fixed the Wifi issue but caused battery life to go down, iOS 8 restored battery life. No problems whatsoever with iOS 9.1 today.
 
iOS 9.1 on my iPhone 5 has been really good. More fluid and snappy than any version of iOS 7 or 8 and without the odd hiccups and random restarts of the early iOS 9.0s. Not a hangup or problem since I upgraded to 9.1.

Just my personal experience. Many videos on the subject on the web, too.
 
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