The posts here are missing context: I'm talking about immutable data structures specifically in the context of UIs. It's common knowledge in the React world tha

The posts here are missing context: I'm talking about immutable data structures specifically in the context of UIs. It's common knowledge in the React world that there is quite a performance improvement using them, because as you change data you no longer have to do expensive checks to figure out exactly what has changed. You just rerender from the top-down and only update specifically what has changed (which just requires pointer checks now). See http://swannodette.github.io/2013/12/17/the-future-of-javascript-mvcs/ http://swannodette.github.io/2013/12/17/the-future-of-javasc.... In general, sure, if you can't take advantage of these semantics then there is going to be overhead and you just gain simplicity. But in certain domains we can take advantage of this simplicity for big performance wins.