Whelp. Turns out if you accidentally install the wrong version of Bonfire, realize, switch to the version you meant to run but forget to reverse the database migrations first... you can get into a situation where it starts recursively generating multiple access control rules on every incoming federation request as they aren't getting found and linked properly. How it was running at all with a couple of billion acl rows I'm not sure, but it is certainly responding a little bit more quickly now I've cleared them out.