Also just so I understand what you’re saying, are you saying that I could intentionally sabotage a server just by subscribing to a lot of communities, which would cause the All page to use a ton of resources whenever people look at it?

On a tiny instance with insufficient resources, maybe. But it's not so much a problem for one person to decide to subscribe to a lot of things. It's more that you have to be careful about eagerly shipping useless messages around big federated/distributed networks. Imagine a world where Lemmy is very successful, and a network of 10,000 instances federate with each other but maybe 9000 of those are tiny personal instances. If one of those servers has an unpopular sub, there's a BIG difference between shipping 50 copies of a post made on that sub to the servers with users that care... vs shipping 10k copies to servers where mostly no one cares. Then multiply that by the potentially hundreds or thousands of posts on unpopular subs across the whole network. It's very easy to ship around millions of messages that no one reads. Good federated designs MUST minimize this.

So if you want to intentionally sabotage the network in the way I'm describing... then you'd set up thousands of lemmy servers and configure users on them to subscribe to everything. People would probably refuse to federate with you and the bad thing would never happen... but yeah... thousands of servers could generate a pathological amount of replication load on the servers hosting active communities.