It seems like the end result of React's diffing algorithm is going to be an appendChild. Exactly what is happening in the vanilla JavaScript. But I don't think the diffing algorithm itself is the reason for the time disparity but rather React's rebuilding of the tree to diff against the DOM. I agree with you, shouldComponentUpdate() should massively speed up React.
Right, with lots of elements, React still needs to render and diff each component which for a lot of elements causes a lot of garbage and diffing work. ShouldComponentUpdate / PureRenderMixin avoids this altogether when props/state isn't changed so this gives a significant boost with lots of elements.
