Our ad provider does not have code that is asynchronous, and running multiple ad units is causing the page to load slow for visitors. I don't think I can get away from the iframe, but I want to make this asynchronous.

How can I change this ad code unit to use ASYNC and be asynchronous?

<script type="text/javascript">
atOptions = {
    'key' : 'Ress2mvPP9MeVFjJL5gi',
    'format' : 'iframe',
    'height' : 250,
    'width' : 300,
    'params' : {}
};
document.write('<scr' + 'ipt type="text/javascript" src="http' + (location.protocol === 'https:' ? 's' : '') + '://www.domain.com/Ress2mvPP9MeVFjJL5gi/script.js"></scr' + 'ipt>');