Angular, white screen appears for one or two seconds before displaying the page

I'm new with angular 4. I have created some components:

and I have applied a bootstrap template to my first project.

I have no error but the problem is that when I navigate between page , a white screen appears for 1 or 2 seconds before displaying the page.

index.html

<body>
    <app-root></app-root>
</body>

This is the app.component.html

<app-nav-bar></app-nav-bar>

<router-outlet></router-outlet>

<app-footer></app-footer>

Project structure

for exemple , I'm in the homepage :

when I click on link about , this white creen appears for 1 or 2 seconds :

and then the page is displayed: