Thanks for your purchase from Zerolemon.
Please note that the fast charge function is determined by the phone and charger not the battery.
So it also needs your phone and the charger support fast charging.
The reason why we suggest turning off the fast charge option is that it will prevent the 10000mah battery from charging to 100%.
Because the charger and the phone are programmed to only charge 100 minutes which can fully be charged the original battery but can't for the 10000mah battery.
But if you are in a rush to go out, you also can use the fast charge function to charge the 10000mah battery, it's just the battery will not be truly fully charged if you do like that.