Jeff Atwood wrote:
USB carries pretty low voltage, it'll be slow for charging anything with a tablet-sized battery.
In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); in USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1500mA (1.5A).
Compare with the iPad 4 charger which is 12 watts (upped from the 10w of the iPad 3 and earlier).
https://discussions.apple.com/docs/DOC-3511
PC USB 5 volts 0.5 amps 2.5 Watts iPhone Charger 5 volts 1.0 amps 5 Watts iPad Charger 5.1 volts 2.1 amps 10 watts