The silent tears of the onload event
It is a public secret that the window.onload event lies at the core of this problem. It will only execute after the whole document, including all page assets like images and objects, is loaded. So if you work with a lot of content, a series of images or a slow server, you will always experience this negative side-effect.
A band-aid while awaiting the real cure
So a document.write('<link rel="stylesheet" type="text/css" href="js_hide.css" />'); would do the trick. We should only load this style sheet when there is enough W3C DOM support to add the actual show/hide behavior.