iDeviceGuru said:USB ports don't produce a high enough output voltage for the iPad to recognise a charge, all the USB port is really for is the transferring of data, USB 3.0 is the faster that 2.0 for obvious reasons, think about it this way, the iPad has a 11,000 mAh battery, most laptops are around the 15,000-25,000 mAh range, with your laptops charger input being about 5v the laptop would loose charge almost instantly trying to charge an iPad.
The iDevice Guru
It's not the voltage , but the ampere rating is too low on a USB port, it requires 2 amps 10watts to charge IE: more power, that's why it takes so long is it is trying to trickle charge the iPad.
Some USB 2.0 ports and accessories do not provide enough power to charge iPad. When this occurs the message "Not Charging" appears in the status bar next to the battery icon.
On most USB ports the power spec is 100mA ,unless the device is configured for high power(charging port), then it is 500mA. The mother board bus voltage is 4.75v to 5.25v.
Laptop power supplies range from 15-20 volts and 3-7 amp rating depending on the size of laptop.