The redesign has absolutely no resource management. They don't load and unload media when its no longer in the viewport either from scrolling or opening a thread, and the redesign infinitely scrolls.

Plus thread view is a modal over the feed unless you navigate directly to its URL, so often times you'll have media loading and running in the thread view while other media may still be playing back in the feed.

I don't think that's the only source of the insane memory leak in Safari in particular (you will get into the GBs if you leave a media-heavy feed open past 10 minutes or so), but its gotta be a big contributor.