As you create more you get better at it. By default, after you've crafted 10 of item you get 5% productivity on that recipe. Then, every time you double the amount you've crafted, you get another 5% productivity. For example, having crafted 250 of an item would give you 25% extra productivity on that recipe (10, 20, 40, 80, 160) and your next bump would happen at 320 crafted.

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