I am new to the fediverse, and I don’t use Friendica, so I could be entirely wrong about this. However, from what is described, perhaps Friendica has some sort of feature in which would trigger your instance to go out to fetch some data from another instance. Someone exploited this feature, spammed your instance with content from assortment of subdomains on the *.activitypub-troll.cf
domain, and most if not all of them are probably non-existent. As result of that, your server is re-checking every 10 minutes to see if they’ve came back online. This would also explain why shutting down the Friendica service resolved the problem for you.
Yeah, the entire setup is quite finicky still. Part of me thinks Fediverse is forced into the spotlight by Twitter (Mastadon) and Reddit (Lemmy), and the whole thing is not quite baked yet. Don’t get me wrong, having a more open space is great, but there are so many things that’s not quite ready for prime time. I hope the dev team behind the platform (not the self hosted instance admins) will be more open to ideas and rapidly improve the platform.