iOS 9 GM on the iPhone 4S

My girlfriend has an iPhone 4S, that I originally bought in the Summer of 2012. Right now, she is on iOS 8.4.1 (I think that is the most current iOS version) and performance is ok'ish.

Is there anybody running iOS 9 GM, with a 4S and if so, how is the performance? Would it be worth it to update the phone to 9 or should I just leave it on 8.4.1?

I've personally been running the Dev Previews of iOS 9 since about DP 2, on my 5S and it's been fine (except for Whatsapp not wanting to connect on mobile) but that is also a much more capable piece of hardware than the 4S. Any advice is greatly appreciated.