It can be stunning the first few times you see this sort of thing, but Haskell run time performance is not that hard to reason about it - is simply different from strict languages. As an experienced Haskeller I could see their point at the definition of the data structure and operations. In fact there is an entire data structure library built around this idea of O(1) snoc/appends, the dlist: https://hackage.haskell.org/package/dlist-0.7.1.2