I wrote a browser extension to collapse HN comments past the first depth and ended up just attaching their css classes and setting the sub-comment count myself. It was the fastest behavior I could find at the time for reasons I've since forgotten. I did try to use their existing js but it took forever to collapse the page.

Here's my collapse code if interested

https://github.com/a13o/disengaged/blob/master/src/hacker-ne...