You are of course correct. I recommend you check out this 5 posts on Persistent vectors in Clojure http://hypirion.com/musings/understanding-persistent-vector-... "Spoiler from part 4":

Transients are an optimisation on persistent data structures, which decreases the amount of memory allocations needed. This makes a huge difference for performance critical code.

http://hypirion.com/musings/understanding-clojure-transients