I tested seven functions that extract all paths through on 3-level, 3 keys per level and 6-level, 6 keys per level embedded map structures. The second Specter version above and miner49r's reduce-kv version are about the same speed, 2X (for the 3x3 map) and 3.5X (for the 6x6 map) faster than any other version. Another illustration of Specter producing code that's as fast or faster than handwritten code. It's very impressive. (The first Specter version is about the same speed if I take out map butlast.) Full details about my tests are available here: https://github.com/mars0i/key-path-tests

@mars0i nice work, thanks for investigating it