That's really not how that works. Transistors have a "latency" measured in picoseconds, not nanoseconds. A 128-bit adder will have a latency of 1-clock cycle,

That's really not how that works. Transistors have a "latency" measured in picoseconds, not nanoseconds. A 128-bit adder will have a latency of 1-clock cycle, a 10% increase from a 64-bit adder of latency of 1-clock cycle. Aka: it doesn't matter cause it'd almost certainly fit inside of the clock cycle of modern CPUs.