FYI, the inline Youtube ads have been temporarily removed because they were causing issues with loading pages. They may or may not return later.
Could positioning them at the bottom (after last post) be viable? In other words, if you can get the thread content to successfully load first, then have the ads coming after.. well, while that is loading, at least we can view the threads without lag? (granted, not sure if ads placed at the bottom poses as a problem of 'visibility' from the advert companies' standpoint).
It is sort of inline thinking with javascript and webpages. Yahoo recommends placing your javascripts at the bottom of your html code (if possible), as javascript loading hangs the process of the page loading / rendering until the script is fnished loading.
EDIT - Either that or implement a more 'light weight' ad system.. if I recall, the youtube ad had side buttons that you could click to display more footage thumbnails... so perhaps this adds weight to the ad system when downloading? If it was static images (no heavier script needed), it might not slow down much at all.