You are correct. _.uniqueId() will absolutely wreck your list rendering performance if it is called on every render. 99% of the time, you can find a better unique identifier, but if you absolutely need something like _.uniqueId(), it should never be called in the render loop. It should be attached to the data being iterated over, in advance.<p>But... the other problem with _.uniqueId is that in an isomorphic setup your server will just keep counting up and up while your client will start from 0 on every page load. So the first time you reinflate React on the client side the checksums won&#x27;t match and will cause a full-page re-render.