Vue 3.5 cuts reactivity memory 56%, adds lazy hydration

Vue 3.5 drops reactivity memory 56% and speeds up large arrays up to 10x with no breaking changes. Reactive props destructure is now stable by default, letting you set prop defaults with native JS syntax instead of withDefaults. SSR gets lazy hydration so async components hydrate on demand—like when visible—plus useId() for stable IDs and data-allow-mismatch to suppress hydration warnings. Custom elements gain shadow DOM toggling, host APIs, and nonce support. Upgrade to clean up props code and fix SSR hydration headaches.