What is the impact of indexes as keys?

Keys should be stable, predictable, and unique so that React can keep track of elements.

In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do.

{
  todos.map((todo, index) => <Todo {...todo} key={index} />);
}

If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.

{
  todos.map((todo) => <Todo {...todo} key={todo.id} />);
}