Hello Hubert,
We apologize for not being able to respond sooner.
The effect you’re having an issue with happens due to the way in which HTML, CSS & JavaScript are loaded. The HTML loads first and JavaScript(which gives the sliding effect) loads after the HTML loads, thus there is re-arrangement of HTML that happens for a fraction of a second.
To resolve this, we would recommend using a page preloader plugin which will make the site loading very presentable for the user.
Hope this helps you.
Thank you
Regards,
Team ProDesigns Themes