A watched pot never boils....
But there is a decent explanation. Your old iPhone 4's battery has degraded over time, so 1% of its battery is probably less energy than 1% of the new iPhone 5's battery.
I have an old iBook G4 at work that charges from 0% to 100% in about 15 minutes. It's not really charging faster; its battery is just shot and only holds a tiny bit of charge. It lasts about that long on battery when you unplug the power!
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I could swear that the iPhone 5 charges much slower than my old iPhone 4. I use a USB wall adapter while i am at work and my iP4 would charge to 100% within a few hours while I listen to music etc. With iphone 5 it is taking like 5-10 minutes to charge 1% :/
Oh, something else I should mention... What kind of USB wall adapter?
If you're using one wasn't made to charge the iPhone, it could be delivering only half an amp or less, causing slow charging. The charger that comes with your phone delivers a full amp (twice the power).
Use that if you can.