Clearly, it depends on the architecture used. From what dang says, it seems like HN renders every single page view dynamically if you are logged in. I guess that has something to do with the voting mechanism; I would have thought that could be handled client-side instead.

Either way, it's not just down to the number of comments on a thread, as you imply, clearly the number of requests for that page plays a part. I'm guessing HN has been getting a huge amount of traffic, probably way more than your laptop could serve dynamic pages at.