Mary Jac Heuman discusses the benefits and challenges of Single-Page Applications (SPAs) in a web development context. SPAs offer flexibility, smooth user experience, and fewer page loads compared to traditional multiple-page apps. However, they also introduce trade-offs such as increased complexity and potential issues with older or less powerful devices. To effectively monitor SPA performance, developers need to track dynamic page elements like animations, API calls, and rendering lifecycles that can interrupt the user experience if there are slowdowns. A user-centric approach by listening to browser events and tracking user interactions over time is recommended. Key aspects of SPA performance include route changes, user interactions, and error tracking, which require monitoring tools to optimize the users' experience.