Weird loading of my website 🤔

Hi everyone and thanks already for looking into my issue, I really appreciate.

The loading of my website, using a template, seems to display the navbar first, then hide it, to finally load the whole page. Here is the link for you to have a look: getpartnrs.com

Any idea how to solve this?

Thanks in advance!

Here is my site Read-Only: LINK
(how to share your site Read-Only link)