Performance boost in some cases

docs/09.7-pure-render-mixin.md

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-If your React component's render function is "pure" (in other words, it renders the same result given the same props and state), you can use this mixin for a considerable performance boost.
+If your React component's render function is "pure" (in other words, it renders the same result given the same props and state), you can use this mixin for a performance boost in some cases.
 
 Example: