In my experience, anything with display:none is not rendered by the browser and doesn't take up any real resources. I've used this trick with infinite side scrolling slide show type interfaces when I found that beyond about 20 elements, things started slowing to a crawl. Setting offscreen elements to display:none fixed it right up.
"Doesn't take up any real resources" is anything but true. An iframe in particular is basically a new render process, which adds a lot of overhead. Just because you've set "display:none" on an element doesn't mean it's free.