The charging speed is determined by the wattage. The output is measured in amperage and voltage, where amperage is the amount of electricity that flows from battery to device, and voltage is the strength of the electric current. When you multiply volts by amps, it gives you wattage.
Each device also has a regulator inside that determines how much power it would like to receive. A phone at 10% battery will charge faster than a phone with an 80% battery.