There was some work recently to improve the performance of persistent data structures. See http://clojure.org/transients http://clojure.org/transients Calling

There was some work recently to improve the performance of persistent data structures. See http://clojure.org/transients http://clojure.org/transients Calling (transient x) gives you a mutable copy of x which is created in O(1) time. You can then use functions like conj! and assoc! to modify the data structure in place. When you've finished, call (persistent! x) which returns you to a pure data structure (another O(1) operation).