title: FlatList hangs UI if uses initialNumToRender for larger value
body: <p>I have created contact screen where on the right side there is a alphabatic dictionary character, if user select any of the character I need to move the flat list to that contact.</p>
  <p>But previously I fount it was not working because flatlist didn't load all the data in the start and if i tried to move to that data which is not loaded by flatlist nothings happen.</p>
  <p>SO for the time being i found out the solution with initialNumToRender internal method in which I send total count, it working but as My records are around 200 it gets hanged and take time between switching</p>
  <p>Is there any better way to achieve the same ?</p>
  <p>I used the flatList like this</p>
  <pre><code>&lt;FlatList
          ref={ref =&gt; (this._mainList = ref)}
          initialNumToRender={this.props.data.length}
          scrollEventThrottle={16}
          onViewableItemsChanged={this.onViewableItemsChanged}
          extraData={this.props.data}
          getItemLayout={this.getItemLayout}
          keyExtractor={item =&gt; item}
          getItemLayout={this.props.getItemLayout}
          showsHorizontalScrollIndicator={false}
          showsVerticalScrollIndicator={false}
          {...this.props}
        /&gt;
  </code></pre>