Minecraft wiki entry for Logic_gates, associated text: But here is the issue: You now need to borrow a carry again, because, in the two's place, there are two ones. This is done by ANDing the output of the first half-adder with the carry from the previous bit and this is a huge issue. Because, for the next bit, you AND the borrowed carry again, and again. Each AND gate takes 2 ticks, so, in order to calculate all of the carries that need to be added up in the final step, it takes 2 ticks times 4 bits, or 8 ticks.