Optional pre-loading module loading, when it's necessary to load the resource at the first page load for some lazy routes. For example, by default, the order module must first access the /order route before it will actually start download resource files. When using PreloadOptionalModules and specifying preload: true, it will automatically download resource files after the Angular project is started.

 providers: [PreloadOptionalModules],
 imports: [
      path: 'order',
      loadChildren: () => import('./order/order.module').then(m => m.OrderModule),
      data: { preload: true }
   ], { 
    preloadingStrategy: PreloadOptionalModules