lazy-header.jsx

import React, { useState } from 'react'
const NavMenu = React.lazy(() => import('./NavMenu'))

function Header() {
  const [showMenu, setShowMenu] = useState(false)
  
  return (
  <div>
  	<React.Suspense fallback={<p>Loading...</p>}>
      {showMenu && <NavMenu />}
  	</React.Suspense>
  </div>
)}