>what else would cause that? How much work the code that they generate does. Clojure is mostly pure right? If you have, e.g. a btree and you change some node

>what else would cause that? How much work the code that they generate does. Clojure is mostly pure right? If you have, e.g. a btree and you change some nodes the old notes won't get deleted, a new tree will be created where some of the nodes point to the still valid old ones and some point to your changes. That's more work then just mutating the variables.