Thanks (and Dylan18607). So to continue assuming, the number of wires specified is just the input/output, and the add.cc has more intermediate "wires"?

It's more like: to implement add you need a ripple carry adder. You can't evaluate bit 6 until you've evaluated bits 1-5. So there's a nested dependence that makes the circuit deep. String cap, on the other hand, can be implemented in parallel by looking at pairs of characters independently. So that makes it into a parallel band of small circuits, which we can evaluate in parallelism. Also I think the capitalization routing affects fewer bits of a char (it's just toggling one bit in each char, IIRC).

It's faster to carry six glasses at once than having to carry them to some point one by one.

That makes perfect sense, thank you! I also didn't know until today that capitalization in ascii is just the 6th bit. I thought it would have been doing 32 subtract operations. (I'll be honest I haven't written my own uppercase function since school haha)