What I don't really get is, if you've implemented PureComponent and shouldComponentUpdate wherever possible, how can adding Immutable possibly improve performan

What I don't really get is, if you've implemented PureComponent and shouldComponentUpdate wherever possible, how can adding Immutable possibly improve performance? The argument is that if you prevent mutation then you can rely on only a shallow comparison of the object references as a reliable test for whether any of the data has changed. Your shouldComponentUpdate implementation becomes a one-line equality test for each prop that might change, or the equivalent. This may be significantly faster (and potentially easier to maintain) than any more detailed comparison of props to decide whether anything significant has changed in shouldComponentUpdate. Of course, this doesn't address the performance implications of maintaining your state in some sort of immutable data structures rather than just mutating it. Nor does it address the performance implications of using a library like React that declaratively renders your content and does the whole vDOM diff algorithm thing instead of just poking the DOM in exactly the required places. Both of those strategies can be orders of magnitude slower than the alternatives, and both of them can cause architectural and maintenance headaches of their own, and so the questions in those cases are whether the performance is still good enough and whether the benefits in other respects outweigh those costs.