Minecraft wiki entry for Mining_Fatigue, associated text: The effect decreases attack speed by 10% × level, equivalent to the attack speed cooldown attribute‌[Java Edition only] decreasing by 0.2 × level. For the in-game default level III, attack speed decreases by 30% (attack speed cooldown attribute decreases by 0.6), and it takes ~370 times longer than usual to break a block with the proper tool for that block.