First when site loading, we see a icon with text "Loading" below that icon, is a GIF added by developers of that site and that animation called as splash screen.
After site is finished loading, we see that some screens come from right and left side because developers have added transition effect on after loading a site. So they have made too many animations in their site. Too many animations and effects sometimes cause site slowness. You can see this when this site is loading first, it takes too much time to start because of too much animation effects, images and GIF of sites.
So what i will suggest to not add too much effects, images, and GIF in your site if you want to get your site's better performance result.
Hope this will help you.