[drfloob]

Part answer to your question, part shameless plug.

Node cloning can be done fairly quickly if the bulk of the node data is shared. The `_tree` immutable tree library (https://github.com/drfloob/_tree#quality-metrics) has a 1,024 child-node benchmark. On my slow laptop, it can build 1024 node trees at ~10/second, and the performance appears logarithmic on node count (http://jsfiddle.net/9x7aJ/2734/). I found it fast enough for my client-side use cases, but Clojurescript's optimized branching scheme could probably be implemented to boost performance. And anyway, if 10ops/second isn't fast enough, `_tree.Tree.batch()` lets you temporarily bypass immutability if needed, making modifications much quicker.