It’s definitely caused/should be mitigated by the theme.
Looks like the theme has a loading animation to hide this, but it was disabled in Layout Hub settings? Also, I guess it was still looking ugly after loading animation turned off.
Though It’s possible to reduce the time this unformatted menu data is shown by slightly modifying the megamenu HTML.
The theme developer has replied to me and told me that This issue happens because the loading of your elements goes faster than the styles and scripts are loaded, you can add this code to theme style.css.liquid.
First of all – it’s theme developer’s responsibility to ensure that site looks reasonable even on slow connections, not yours.
The stylesheet is loaded and applied pretty fast; the problem is that megamenu element has no proper class assigned initially, so no styles apply to it. The class is only assigned by the scripts.js javascript and that one is only run on “load” even, which is pretty late for such a critical page element.