I was previously seeing this error. After optimizing my code, I no longer see it. I figured out the problem by adding console.log() statement to the render() function of the Component that creates the FlatList, and the function that renders each item in the List. I noticed that my code was previously re-rendering the entire FlatList and all its items whenever there's a state change to any component on that page (even a component that's not related to the FlatList). I fixed this by converting various components to PureComponents. Here's what my FlatList declaration looks like:

<FlatList
    ref={(ref) => { this.flatListRef = ref; }}
    data={allPosts}
    initialNumToRender={7}
    renderItem={({ item }) =>
      <Post postJson={item} isGroupAdmin={isGroupAdmin} user={user} />
    }
  />

Notice that I'm returning <Post /> which is a pure component:

import React, { PureComponent } from 'react';
class Post extends PureComponent {

  render() { ... }
}

This ensures that the FlatList re-renders a only if the post changes. When I was previously passing a normal function to renderItem i.e., a function that does something like this:

return (
  <View>
  ...
  </View>
);

I noticed that the FlatList was re-rendering all items whenever any item changed. Now, by using a PureComponent, the FlatList only renders the new item added to the list (if the list is already being displayed).

It still takes relative long to render the entire list the first time. However, initialNumToRender ensures that the screen is filled up pretty much instantaneously (while the remain items get rendered in the background). And more importantly, after that initial rendering, the FlatList only ever has to render one item at a time (the item that changes).

I found this post very helpful).

I've just realized this is also explained here

I noticed that the answer to this question dosen't proffer solution for those using functional component and hooks. I encountered this problem and i was able to get rid of it by using the hook "useMemo()"

<FlatList
                keyExtractor={keyExtractor}
                data={productsState.products}
                renderItem={renderItem}
            />
const renderItem = ({ item }) => (
            <ListItem
                title={item.ProductName}
                subtitle={(item.ProductQuantity) + " " + (item.QuantityType !== 
                null ? item.QuantityType : " ") }
                bottomDivider
                topDivider
                chevron
                checkmark={checkMark}
                onLongPress={() => setCheckMark(!checkMark)}
                rightSubtitle={(item.Currency !== null ? item.Currency: " " ) + 
                " " + (item.productCost !== null ? item.productCost: " " )}
                rightSubtitleStyle={{ marginTop: -20 }}
                badge={{ value: item.sellingPrice, textStyle: { color: 'orange' }, containerStyle: { marginTop: -20 } }}
            />
        )

The renderItem function is an expensive computation, because it a long list to render. Instead I memoize it as follows

            const memoizedValue = useMemo(() => renderItem, [productsState.product]);

<FlatList
                keyExtractor={keyExtractor}
                data={productsState.products}
                renderItem={memoizedValue}
            />
const renderItem = ({ item }) => (
        <ListItem
            title={item.ProductName}
            subtitle={(item.ProductQuantity) + " " + (item.QuantityType !== 
            null ? item.QuantityType : " ") }
            bottomDivider
            topDivider
            chevron
            checkmark={checkMark}
            onLongPress={() => setCheckMark(!checkMark)}
            rightSubtitle={(item.Currency !== null ? item.Currency: " " ) + 
            " " + (item.productCost !== null ? item.productCost: " " )}
            rightSubtitleStyle={{ marginTop: -20 }}
            badge={{ value: item.sellingPrice, textStyle: { color: 'orange' }, containerStyle: { marginTop: -20 } }}
        />
    )

Don't forget to import useMemo from react, inorder to make this work.

Good Luck!

If you are using a functional component, wrapping the component in memo is a good way to prevent unnecessary renders without going through the hassle of converting a functional component to a pure class component. This post explains it more

follow this example:

In the parent component:

import React from 'react';
import {FlatList} from 'react-native';
import PostCard from './PostCard';

export const NewsFeeds = props => {
      return (
        <FlatList
          data={data}
          initialNumToRender={4}
          refreshing={loading}
          renderItem={_renderitem}
        />
      );
    };

const _renderitem = ({item}) => <PostCard item={item} />;

In the child component

import React, {memo} from 'react';
import {View} from 'react-native';

const PostCard = (props) => {
        return (
            <View>
    
            </View>
        );
    };
    
 export default memo(PostCard);

If you are using a class component, make sure your component is a pure component by extending React. PureComponent in your class definition

class NewsFeeds extends React.PureComponent {
  render() {
    return (
      <FlatList
          data={data}
          initialNumToRender={4}
          refreshing={loading}
          renderItem={_renderitem}
      />
    )
  }
}