Well, actually.. big instances like kbin.social (but that is the same with other big instances or software), need to process not only the outbox but also the inbox of the activitypub. So this thread with comments, need to be sent to several instances. And each instance need to process this comment now. And the other way around is also true, I created this thread but that needed to be sent towards multiple instances (which is converted the inbox from their server perspective). Kbin.social had mainly issues with sending out messages actually (the outbox), because many instances are not responding or are blocked, causing retries and eventually also an increase in the queue.