iOS 9 Beta

[Kintaro]

I installed on my iPhone 4s and is fast, maybe a little faster than 8.4.1.

 

[JVCode]

I updated my iPhone 6 Plus and my iPad Air 2 and it sucked the battery life from my iPhone, so I decided to revert back to iOS 8. Have you noticed any significant drain in battery life after upgrading to 9?

 

[Robru]

No problems with the battery life (iOS9 beta 2)

 

[Kintaro]

I solved my battery problem (present on iOS 8.4 too) disabling Background App Refresh

 

[rafass]

I solved my battery problem (present on iOS 8.4 too) disabling Background App Refresh

So, 8.4.1 is running good in 4S? on this video looks slower:

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[Gene Steinberg]

The promise of iOS 9 is improved performance on older hardware. While I couldn't comment due to the fact that I'm a registered Apple developer, I think there's enough feedback out there on the first few betas and performance issues.