What is the Fast Weapon Upgrades option?

When you turn this option on, you will notice that "+" and "-" buttons will appear beside the purchase button on weapons shown on the Weapons menu. This feature will allow you to upgrade weapons up to 20x per purchase.

Tapping the "+" button will increase the amount of that upgrade you would like to purchase and the "-" button will deduct this value. When you are ready, you can now tap the center purchase option to buy that many upgrades in one go.

You can turn this feature on or off by tapping the circled icon on the right of the respective option. If a green tick is shown, it means that the feature is on.