Huge numbers of users don't really mean that much. Bandwidth is the main cost, but that's kept low by having a simple design.
Serving the same content several times in a row requires very few resources - remember, reads far outnumber writes, so even dynamic comment pages will be served many times in between changes. 5.5 million page views a day is only 64 views a second, which isn't that hard to serve.
As for the writes, as long as significant serialization is avoided, it is a non-issue.
(The vast majority of websites could easily be designed to be as efficient.)
There is some caching somewhere as well, probably provides a bit more boost.
I've been at my work laptop (not logged in) and found something I wanted to reply to, so I pulled out my phone and did so. For a good 10 seconds afterwards, I could refresh my phone and see my comment, but refresh the laptop and not see it.
That is great ! :D
>It's a lot.
Well yes, but HN isn't really static though. Fairly Dynamics with Huge number of users and comments. But still, I think I need to rethink lots of assumption in terms of speed, scale and complexity.