useFPS.js

  const ref = useRef()
  const items = useStore((state) => state.items)
  const [fast, set] = useState(true)
  const Component = fast ? ItemFast : ItemSlow

  // Kick off a render-loop which mutates all state objects/each frame
  useEffect(() => {
    let lastCalledTime = Date.now()
    let fps = 0
    function renderLoop() {
      let delta = (Date.now() - lastCalledTime) / 1000
      lastCalledTime = Date.now()
      fps = 1 / delta
      ref.current.innerText = 'fps ' + fps.toFixed()
      useStore.getState().advance()
      requestAnimationFrame(renderLoop)
    }
    renderLoop()
  }, [])