For a followup, with a solution for faster charging on many Windows PCs, please see: Now we’re chargin’ with gas: ASUS Ai Charger.
USB 2.0 ports are, according to the specification, limited to providing 500 mA of current to connected devices. The Apple iPhone charge provides 1000 mA, so it can charge an iPhone about twice as fast as a standard USB 2.0 port.
USB 3.0 increases the current limit to 900 mA. Even though the new Lightning-to-USB cable is a USB 2.0 cable, I was hoping that it would take advantage of the higher current limit when connected to a USB 3.0 port. After all, there are USB 2.0 ports on Apple computers and some PC motherboards that are capable of providing higher currents to iPhones and iPads.
Unfortunately, this is not the case:
The charging rate when connected to a USB 3.0 port is basically identical to USB 2.0, and much lower than the wall charger.
These tests were conducted on my iPhone 5 within a few days of release day. My procedure was to deplete the battery, using the Drain My Battery app, until the phone shut off. After a couple of minutes to cool off, I connected the phone to a power source, and treated that time as a 0% charge. When iOS was back up and running, I used the Battery Magic app to monitor the charge percentage. I recorded the time and charge percentage manually at irregular intervals of a few minutes, until the app reported 100% charge.
The wall charger is the small, cubical 1 A charger included with the iPhone 5. The USB 2.0 port was a back-panel motherboard port on my EVGA X58 SLI LE motherboard. The USB 3.0 port was a back-panel port on a Syba SD-PEX20122 PCI-Express card in the same system. The USB 3.0 card is properly connected to a molex power cable. Neither of the USB ports claims to have any special, high-current or iThing-specific modes, just the normal 500 mA and 900 mA limits appropriate to their versions of USB.