It is not lag, it's more like your server simply doesn't know about the content yet because it didnt federate yet.
Say we make 2 servers, a.example and b.example and theres a community on a.example that is quire active.
now a user from b.example opens it, lettings its server know for the first time that the community even exists at all! b.example will not ask a.example for all the posrs on the community because, especially later, there'd simply be tol much, so your server only shows what it knows about.