shallowRef, error boundaries, CLS fixes, layout shift, and every trick that actually moves the needle. Who This Is For You're building with Vue 3 + Nuxt 4 and your app works — but Lighthouse isn't happy, users feel the jank, and you've run out of obvious things to fix. This guide goes beyond "add v-memo and call it a day." We'll cover reactivity tuning, rendering strategy, layout stability, Core Web Vitals, and Nuxt-specific patterns that most tutorials skip. 1. Reactivity: Use the Right Primitive Vue's reactivity system is powerful but easy to misuse. Choosing the wrong primitive is the #1 hidden performance killer. shallowRef — Your New Best Friend ref() makes every nested property deeply reactive. For large objects or arrays you don't need to observe deeply, this is expensive.…