I've noticed that my iMac at work (21.5", Mid 2011) charges my iPhone 5 about 2-3x faster than the included wall charger.
How does it do that?
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If you have the USB 3 SuperSpeed (on your Mac) it can deliver up to 100 Watts.
For iPhone it will deliver 10 Watts (5 Volt x 2 Amps) which is more than your charger.