Why 1 A cable charging to 2.1A or 2.4 A device(iPad) is slow

Because both are replying on the detection voltage signal (D+D-), and determine the charge current when charging Apple devices.

When using cable 1A to 2.1A or 2.4A charge the device, and the cable  is bound to increase the pressure drop (no cable
change, the current increases, the voltage drop becomes large), the relationship between pressure drop and inversely proportional to the voltage of just greater the pressure, the output voltage will be lower, the lower the voltage at the output, the two signals (D +, D- ) assigned above the voltage will be lower; in order to ensure the normal charge the device, so be sure to get the two signal cable nominal voltage value (such as D + D- voltage 1A or 2.1A / 2.4A ) However, when all the peripheral devices (such as cable) can not be changed, only reduce the pressure drop caused by the wire to meet the requirements, the voltage drop proportional to the current, smaller current drop is smaller, the resulting cell phone the current is smaller, and the charging time will be longer, slower charge rate.