Prevent rerender on function prop update

1. Use React feature React.memo for functional components to prevent re-render if props not changed, similarly to PureComponent for class components.
2. When you pass callback like that:

<Section
    ...
    updateSection={value => updateSection(idx, value)}
/>

your component Section will rerender each time when parent component rerender, even if other props are not changed and you use React.memo. Because your callback will re-create each time when parent component renders. You should wrap your callback in useCallback hook.

3. Using useState is not a good decision if you need to store complex object like initialForm. It is better to use useReducer;

Here you could see working solution: https://codesandbox.io/s/o10p05m2vz