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<p>If your <a href="/react-native/docs/next/flatlist.html"><code>FlatList</code></a> is rendering slow, be sure that you've implemented <a href="https://facebook.github.io/react-native/flatlist.md#getitemlayout"><code>getItemLayout</code></a> to optimize rendering speed by skipping measurement of the rendered items.</p>
<h3><a class="anchor" aria-hidden="true" name="js-fps-plunges-when-re-rendering-a-view-that-hardly-changes"></a><a href="#js-fps-plunges-when-re-rendering-a-view-that-hardly-changes" aria-hidden="true" class="hash-link" ><svg aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>JS FPS plunges when re-rendering a view that hardly changes</h3>
<p>If you are using a ListView, you must provide a <code>rowHasChanged</code> function that can reduce a lot of work by quickly determining whether or not a row needs to be re-rendered. If you are using immutable data structures, this would be as simple as a reference equality check.</p>
<p>Similarly, you can implement <code>shouldComponentUpdate</code> and indicate the exact conditions under which you would like the component to re-render. If you write pure components (where the return value of the render function is entirely dependent on props and state), you can leverage PureRenderMixin to do this for you. Once again, immutable data structures are useful to keep this fast -- if you have to do a deep comparison of a large list of objects, it may be that re-rendering your entire component would be quicker, and it would certainly require less code.</p>
<p>Similarly, you can implement <code>shouldComponentUpdate</code> and indicate the exact conditions under which you would like the component to re-render. If you write pure components (where the return value of the render function is entirely dependent on props and state), you can leverage PureComponent to do this for you. Once again, immutable data structures are useful to keep this fast -- if you have to do a deep comparison of a large list of objects, it may be that re-rendering your entire component would be quicker, and it would certainly require less code.</p>
<h3><a class="anchor" aria-hidden="true" name="dropping-js-thread-fps-because-of-doing-a-lot-of-work-on-the-javascript-thread-at-the-same-time"></a><a href="#dropping-js-thread-fps-because-of-doing-a-lot-of-work-on-the-javascript-thread-at-the-same-time" aria-hidden="true" class="hash-link" ><svg aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Dropping JS thread FPS because of doing a lot of work on the JavaScript thread at the same time</h3>
<p>&quot;Slow Navigator transitions&quot; is the most common manifestation of this, but there are other times this can happen. Using InteractionManager can be a good approach, but if the user experience cost is too high to delay work during an animation, then you might want to consider LayoutAnimation.</p>
<p>The Animated API currently calculates each keyframe on-demand on the JavaScript thread unless you <a href="https://facebook.github.io/react-native/blog/2017/02/14/using-native-driver-for-animated.html#how-do-i-use-this-in-my-app">set <code>useNativeDriver: true</code></a>, while LayoutAnimation leverages Core Animation and is unaffected by JS thread and main thread frame drops.</p>