useDeferredValue is a React hook that optimizes performance by prioritizing the rendering of important UI elements over less important ones. It does this by allowing React to interrupt low-priority renders if a higher-priority update occurs, preventing the UI from becoming unresponsive. This hook is best used with components that require complex calculations or slow data fetching. In React 19, useDeferredValue gets the ability to specify an initial value, which can speed up initial renders.