EDIT: 11/2017 — This post was originally published 04/2016. Yehuda Katz now has a WHATWG proposal for “DOMChangeLists”, which is very similar to the behavior of Node#mergeWith() described later in this post.
Proxies are surprisingly fast in general, the code in V8 is relatively simple. It’s hard to say more unless I know what you want them to be compared to? I only mentioned it because I’ve found people assumed, just as I did, that “boy I bet your polyfill is slow cause Proxies likely are”.