I can access the header and navigation instantly by pressing the home key on my keyboard, or near-instantly by flicking my phone's screen downward a few times. Having a navigation header appear is like reading a book that covers half my eyesight with a hologram of the index every time I look at the previous sentence.

I read from the top of the screen and scroll the page so that I'm always reading at the same position without much vertical eye movement, and occasionally flick back a sentence or two for context, which is why it's annoying for me. For a reader who read down the page and occasionally scrolled it would be much less annoying.