How fast a power bank can charge your device is not determined by its capacity but rather, the output charge. The higher ampere it has, the faster it will transfer power to your device making 2A more efficient than 1A and 2.1 A better than both of them. However before you buy one for yourself or someone else make sure that their phone supports quick charging because if they don't then there's no point in having a high-output charger at all!