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My wife has an iPhone 3G, I have an iPhone 4.

Ever since she upgraded to iOS 4, her phone has been much slower than it was on iOS 3.

My iPhone 4, upon upgrading to iOS 5, isn't slower. Instead it has greatly reduced battery life and gets physically warm for seemingly no reason.

What do these two things have in common (besides being upgraded to a firmware designed for the next generation of hardware)?

Both phones can be temporarily healed of their maladies by being Double Hard Reset.

According to Cult of Mac:

To perform a Hard Reset, press and hold the Home and Sleep/Wake buttons on your iPhone simultaneously for 5 to 10 seconds. After a few seconds the Slide to Power Off notice will appear, ignore this and keep holding down both buttons. In another few seconds the device should abruptly turn off

They go on to say that the device will reboot itself after you've done this, but both of my devices stay off.

To complete the Double Hard Reset, you need to power on the phone after you've done this and do it again before the phone gets to your lock screen.

I have personally experienced this helping both phones with their various issues numerous times.

Why does this help? What does doing a Double Hard Reset do to the phones?

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Would you be able to connect one of the devices to Xcode to see what is happening in the iPhone console log when you force the second reboot? Perhaps killing it during boot is forcing some checks that are normally suppressed, but make a difference to your situations? – bmike Jan 23 '12 at 21:22
I only have Windows machines, if I understand correctly, you need a Mac and developer tools to use Xcode. Am I right? – Kalamane Jan 23 '12 at 23:11
On the face of it, there's nothing in common with the 3G and 4 in terms of the hard reset. Each has different RAM, CPU, logic board and firmware. Plus, there's way too much going on in iOS 5 that could cause the phone to consume battery, so this is a question that I would say is unanswerable in its present form. – user479 Jan 24 '12 at 19:47
It's not asking why the Double Hard Reset fixes the issue with iOS 5, it's asking what the Double Hard Reset does that's different than a normal reset. – Kalamane Jan 24 '12 at 20:58

From what you're saying, I don't think you're doing the reset correctly. You're supposed to keep holding the Home and Sleep buttons until the Apple logo appears once more. The phone is not coming back up because you're stopping before the reset is completed.

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