The thing about federation is that every server basically copies any content that the users on it want to see. So if a comment/post is made on lemmy.world, lemmy.world sends out an update to every other server on which a user is subscribed to the thread/community/user. So each instance that has a subscribed user ends up having to process the new comment/post. If a Meta community came in with say, a million users, now every instance has to process the comments for all those users (that is, if folks on those instances want to see that Meta content).

It is a bit inefficient, but it’s just the way a decentralized network has to function. I could see many people thinking that any time you open a thread, the data comes in from the originating instance, i.e. that your home instance doesn’t store the data you are viewing. It is unfortunately, and I think it will be a problem in the future as communities grow.