Accessed in Appearance → Customize → General settings → Page preloader.
Page preloader works as a mask for the time while the layout of a page is not ready yet and it(page) is not useful.
It can be useful in some cases, but generally, it is rather an effect than a real feature.
You can set few things about preloader: background color & image, animation and when should preloader fade.
So if you suspect such thing it is a good idea to deactivate it before debugging.