Right now federation traffics only have minimal impacts to Lemmy. They mostly consume network resource (to send out activitypub messages already waiting in the queue), unlike actual user traffics that consume a lot more CPU resources and database access.
When federation traffics finally become large enough to cause issues on popular instances, I think it should be easy enough for the devs to address (e.g. offloading activitypub subscription to relay servers). Actual user traffics are much harder to scale.
Deebster