This is normal behavior for any web site which uses JavaScript and Css and so on.
It's especially apparent on slow connections, or from slow servers, where the javascript takes a long time to arrive, and start running, and the browser has already rendered the html.
On solution is to add the css class, nt-hidden to the body, and then remove it (using javascript). but some widgets don't like working on "hidden" objects, so there can be side effects to this approach.
cheers
Bruce