I just realized the structural equality semantics could be sped up significantly if equiv() is implemented for the non-abstract types. There are happier paths the code could take: • When comparing vectors, nth is called for every element (worst case), but nth always calls arrayFor which isn't reused • When comparing maps, the map casts itself to a sequence first These cases can go through reduce and reduce-kv respectively and give dramatically different results. Should I open a ask.clojure issue for it?