Well, it really comes down to how many subscriptions there are.
A small instance may only sub to 100 communities, so it is not too bad.
But on the flip side, it means that the big instancr needs to send everything to a huge number of small instances.
In practice I do not think it will be too bad, there will be a set of medium sized instances that most will be attracted to, and they will have the p80 of communities subbed. Smaller ones will be for more technical people who will not worry that they need to ensure the content is subbed to, as they will understand how it works.
I think over time, services that aggregate community details will spring up, and be incorporated into the lemmy search, so it is easier to find things across the entire fediverse, not just your instance. I think there will be a large set of muggle-type user improvements over the next couple of months.