I don’t buy this. My impression was that the slowness was sustained during the entire outage and then instantly let up once Facebook came back online.

I’m really curious why this was the case.

HN tends to slow down when there are very active discussion threads. The FB is down threads died down quite a bit when their site came back online. The threads for FB is back online had much sparser discussion.

There's not a whole lot of mystery here.

Those threads weren't particularly active. They aren't even the most active threads in the last month: https://hn.algolia.com/?dateRange=pastMonth&page=0&prefix=tr...

That said, I can't claim that those threads didn't have more readers than other threads. But I can't say they they did either.

It also seems uniquely interesting that some people experienced the slowness but others did not.

If you loaded in private browsing (or logged out), HN was speedy as normal. If you're like me, from time to time you accidentally sign out and don't notice it until much later.

BGP side effect, like maybe FB DNS normally is answering (for some geographies) DNS queries reaching out to news.ycombinator.com ?

At the time, people were saying the slowdown went away if you logged out. I wanted to comment on things, so I didn't try it.

Trackers?