When you first subscribe to a community it only pulls in the last 20 or so posts and I think a limited amount of comments, and then everything going forward. This seems to be a common point of confusion for a lot of instance admins.

Presumably this is to prevent a possible DDoS/performance failure vector as it would be trivial to setup a large swarm of instances on tiny VM's and then simultaneously start hitting massive communities from a single instance and requesting a large body of historical content.

Edit: Also when you first setup and start subscribing to a large number of instances, this initiates a LOT of communication and database writes. Lemmy still has some performance bottlenecks. Once everything is initially synced and settled it runs fine. I have a friend running their instance on a $5 Linode instance that only has 1vCPU and 1GB of RAM without any issues, and they're hosting users.