Different charging speeds result from the devices' integrated charging electronics, which regulate the optimised current intensity themselves depending on the requirements and charge status of the battery. The charging speed can be influenced by the current output provided by the charger or by the cable used.
USB chargers and power banks often only provide a current output of 1 A. This example* illustrates the difference to many InLine products with 2.1 A or 5 A output using a smartphone with a 2,500 mAh battery: