You shouldn't really be doing this as it's not recommended, but I understand your need :) Unfortunately extensions come quite late on the page loading order, so I don't think that you will be able to get rid of the little delay. I believe there are use voice requests to move extensions loading time to an earlier stage of the page lifecycle, but not sure when that will be released.
Yes, I am aware that I shouldn't do that but the goal with this page is show it in a tv-screen and the customer doesn't want either the name of the page nor the navigation shows.
I created a new extension using top placeholder and inside the _renderPlaceHolders() method I call the custom css. It seems to work but I am not sure if it is because the browser cache or the top section of the page actually hides as soon the page start rendering.