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?