I have an issue with my website. When I enter into it for the first time or when I do a full refresh (ctrl+shift+r), there is a latency, a moment of floating, when all modules and elements appears briefly on screen and then start their animations. What is it due to?

I've made a video to show this issue. This is a wetransfer link: https://we.tl/t-2IenPA5721

Link to my website : https://arwelkenraedt.projetdesite.be/index.php

Thanks in advance!