Trouble wth page content jumping on load

Hello
i feel bad but ive searched and searched for a solution, as i know this is well documented by now. But, i just cant seem to identify with an inspector what it is that has the padding and how i can try to fix it in the custom css.
my page is weyerhof.at . it happens on each page it seems. Id love to be able to fix this without resorting to a page pre loader.

thank you very much

 

also would like to add that its a custom header and sticky. Im not sure the header itself jumps, or if its the page content. Generally its the first set of images at the top that start at the top, and then jump down as the page finishes loading.
again, any guidance would be greatly appreciated. Our new page is finished and this is the last main issue and its kind of driving me nuts.
thank you