They&#x27;re really not though. Java has ConcurrentHashMap and isn&#x27;t slow. Clojure has its persistent HashMap and it isn&#x27;t slow. Scala has, TWO (?) concurrent hash maps in its core and neither is slow.<p>You know, two thirds of the way through the second decade of the 21st century, it&#x27;s okay to spend a few cycles not corrupting your data structures.