There's a small period where the page renders before the navigation bar is visible, so if you're paying attention to the header you can see the navigation bar "pop-in" to the page after load.

I had a quick chat with Mathew May on the dev chat who indicated this is done because we don't know the page size ahead of time and want to collapse items into the "more" menu when they are overflow.

They suggested this could be mitigated with a loading placeholder that is then filled in.

I wonder if you could remember the navbar size from the last request (since it's not very likely the page will change in between them) and use that info to prerender the right amount of navbar items, however you would have to store that info in a cookie and keep it up to date.