On enwiki, the page [[en:Portal:Featured sounds]] embeds 380 media files. Something within TimedMediaHandler causes my browser on my laptop (Firefox 16.0.2, Linux) to grind (100% CPU, unresponsive) for about 10 minutes at around the time the DOMReady or onLoad event fires, apparently while generating the UI for all those embeds. Others on IRC reported similar experiences or outright crashes in Firefox and Chrome. In further testing, a page with 90 embeds seemed to be handled fine for me, and a page with 180 ground for about 15 seconds.
If this can't be fixed directly, perhaps TMH can process the embeds on the page in batches (waiting for one batch to complete before the next runs) or just refuse to process over a certain number of embeds in a page.