Consider that transmission gated look-through may be significantly faster than combinatorial look-ahead. This is not a ripple scheme because time consuming work depends only on A vs B. Multipurpose flag EQ then passes to the left with almost no delay. A technique as old as Konrad Zuse's first relay computers, now almost forgotten:
Might help understanding to imagine the borrow chain alone, as a magnitude comparator. Classic 74F85 takes 14nS to do similar work. I think this comparator might even work backward? Though logic lows in the reverse direction (to the right) might appear as open. Intended normal use would flow to the left, and lows would be GND, or whatever level within reason you might prefer as 0. I'm sure it would work just fine upside down too.
Second: What-if 74181 were redesigned in MUX? With look-through Carry and Zero. Not saying 74181 compatible (if I've revengineered the original correctly) makes ideal use of MUX. But I wanted something familiar to compare. Definately learned a few things from that confusing excercise. Concluding that look-throughs should run circles around both F182 Lookahead and F21 Zero. Let us speak of dread 181 no further...
Relays, where it all began. This slice actually works, if you bang it on the bench a few times to unstick it. My bad for fishing "perfectly good" relays out of the trash. Hot gluing them dead-bug to a popsicle stick. XOR across the coil also wants a diode bridge to absorb flyback. I've got 18V MOV in that duty now on the daft theory it would soak up more energy and allow contacts to open faster. But it made no hill of beans.
Already you see nearly instant lookthrough Carry and Zero. But difficult to make a convincing presentation for exceptionally fast propagation, while waiting two mechanical steps before any of that magic can give a result.