React applications can encounter performance issues due to unnecessary render cycles, where components re-render without changes to their props. This can lead to slow component mounts and deep component trees, negatively impacting the user experience. The "why-did-you-update" library is a tool that helps developers detect these unnecessary renders by monitoring React applications and logging instances where components re-render without prop changes. Developers can address these issues by utilizing React's PureComponent, which only re-renders when props change, or by implementing the shouldComponentUpdate method to prevent unnecessary updates. The tool is best used during local development, while tools like LogRocket provide insights into performance issues in production environments by recording user interactions and app performance data. Overall, understanding and addressing unnecessary render cycles is crucial for optimizing React app performance, and various tools and techniques can assist developers in achieving this goal.