It will be loading order issue - first, the browser downloads document and display it. Later when it loads entire javascript it hides the content to provide the nice animation. So I think it's also affected by internet connection speed.
Solution 1:
hide the content with css so it will immediately load as hidden
Cons: if javascript crashes, the content will stay hidden
Solution 2:
Place this code at the end of body, but before any other scripts are loaded (before any other script tag)
var wows = document.getElementsByClassName('wow')
for (i = 0; i < wows.length; i++){
wows[i].style.visibility ='hidden';
}
It will be executed right after the page loading even before jquery and it should be fast enough.
Solution 3:
Use immediately inserted css file to the head of the document as suggested there
http://robertnyman.com/2008/05/13/how-to-hide-and-show-initial-content-depending-on-whether-javascript-support-is-available/