Comment by _bxg1

As a rule of thumb: anything done in a pure-functional/immutable style is probably going to take a performance hit. This tradeoff is often worth it for maintainability, especially if you're using true immutable data structures like Immutable.js or Clojure's structures which reduce copying to the absolute minimum. But always be aware of this tradeoff. If you're doing a simple operation over 15,000 of anything, just use a for loop for goodness' sake.