There are many lemmy instances in the world, but currently most people are using lemmy.world. This is why everything has gotten so slow.
You don’t have to delete your lemmy.world account, but check out https://lemmy.fediverse.observer/map it’s a geo-based map of lemmy instances – explore stuff nearest you, pick one, sign up, search , subscribe and begin interacting with your favorite communities. It’s easy, free and it will be faster. Try it!
How is this related to instance size? There are large instances which defederate (I think most do), and small instances which do not.
If anything, I’d see it as an argument for joining small ones. There, your voice can have a bigger impact on federation decisions.
Mostly I think if you care about federation status, be sure to join an instance with a federation policy which you like.