Monitoring performance is essential for detecting post-development issues in an application. It helps understand how the application works on different devices, browsers, and users, and identifies slow load times, errors, and other performance issues. To monitor performance in a Next.js application, Sentry's Next.js SDK can be used to track errors, performance issues, and other critical events. The process involves creating a new application on the Sentry platform, installing the Next.js SDK, configuring it with Sentry, and setting up API routes to track performance issues. Sentry's dashboard provides tools for monitoring performance, tracking transactions and events, and identifying issues such as slow load times, errors, and user misery. By using Sentry, developers can effectively monitor performance, identify issues, and fix them accordingly, ensuring the application meets its intended use case.