the speed of your USB cable.
The speed of the USB cable has no bearing on the charge rate. The charge rate is dependent on the power source. Sources that provide more current (they all provide 5 volts) will allow the device to charge faster. The amount of current the device will use to charge is limited by the device. If the device will take 2 amperes and the charger provides 4 amperes, the device will still only use a maximum of 2 amperes. If the charger provides 1 ampere the device will use only one ampere to charge and will thus take twice as long to charge as the charger that provided 4 amperes.
USB ports have a maximum current of 0.5 amperes. A wall charger, such as used for an iPad will provide 2.1 amperes, an iPhone charger will provide just 1 ampere, third party chargers may provide more current. You should be able to charger an iPhone using an iPad charger at almost twice the rate of using the iPhone charger depending on the current limits within the phone.
However, the speed of the cable is irrelevant. When you provide power (voltage and current) there is no such thing as speed as there is no data involved.