The person you are replying to is talking about things like maps implemented with persistent trees. Updating a value in such a map generally only involves copying O(log N) nodes in the tree: It's not necessary to shallow copy the entire tree itself.
> Persistent data structures are different, as their performance improvements are passive. Although seamless-immutable does not (and cannot, while maintaining its backwards compatibility with vanilla JS collections) use things like VLists under the hood, its cloning functions—such as merge—only bother to make shallow copies, as shallow and deep copies of immutable values are equivalent. In practice, this simple passive optimization has been sufficient; we have yet to encounter a performance problem that Bagwell-style persistent data structures would have solved.
from: http://tech.noredink.com/post/107617838018/switching-from-im...