It depends how demanding the requests are. Just as one example, when I request the list of my own threads (to be sure to respond to replies I may have missed through regular browsing of the site), I OFTEN find that that times out. This is probably more likely to happen to a user with many subscribed threads than to one with few, but I see it happen a lot. I can hardly ever successfully request my list of submitted threads.
But perhaps that is just me. My use pattern here is at the extreme end of the distribution.
I don't consider myself a particularly frequent poster, but I see the same frequent timeouts when trying to load my threads page. It's the most important page for me personally, because I come here for the conversation with a highly intelligent group more than anything. Nothing else compares to HN's simplicity and community.
But perhaps that is just me. My use pattern here is at the extreme end of the distribution.