This is the problem we are having with fast growth on a few select communities. The largest servers are being bogged down simply because the software has not been tuned for these large types of instances yet. ActivityPub works best (in it’s current state) by spreading users over smaller/medium sized instances. Folks need to take a look at other instances (and I agree it is hard to find them for a newcomer). You can look at https://fedidb.org/ to look at instances that have been indexed running kbin, lemmy, and other software.

Joining a smaller instance means that your server is not being bogged down by tens of thousands of other users trying to pull updates to their devices at the same time. You can still see the content from other instances, and in many cases it is more reliable because your smaller instance actually has the resources to handle pulling in the posts you want to see. The server-to-server communications that make content federation possible are less resource intensive than pushing updates to user devices. Less users on an instance = more server resources to actually federate content. In the future I am sure instances like lemmy.world will be able to handle the user traffic and federation traffic smoothly, but for now the best way to ensure stability is to join a smaller instance.

(Plug for my instance: https://remy.city, a general purpose Kbin instance. I set it up for personal use but anyone is free to join me in using it. I have defederated from the instances with more extreme viewpoints userbase-wide (like lemmygrad and exploding-heads), and from lemmynsfw.com because of content hosting concerns. I’m open to suggestions on others.)