Performance optimization in UX design should be a continuous consideration, not an afterthought. It's vital to begin during the initial planning and architectural phases to lay a performant foundation, preventing costly reworks later. Throughout design and development iterations, regular performance testing ensures features are built efficiently and don't introduce lag or jank, especially for interactive elements and critical user flows like checkout processes. Optimization becomes paramount before launching any product to ensure a smooth first impression and retain users who often abandon slow sites. Furthermore, it's essential post-launch when analytics reveal user drop-offs or frustration linked to slow loading times, responsiveness issues, or high resource consumption, particularly benefiting users on older devices or with slower network connections.