I think the real problem is world is getting too big. They are by far the largest instance by users, content and post volume. The federation bandwidth requirements (and front end serving requirements) have got to be insane with 7200 people actively posting, with thousands more federating in. It exposes all the cracks that lemmy inevitably has in it's underlying data handling.
It's more to do with the lemmy itself as a platform handles than bandwith. Basically, there is only one channel between any two instances, and it's serial, and each step requires multiple handshakes to complete. Add in geographic distance making those handshakes take a significant part of a second to complete, and you end up with a single channel that gets flooded.
Blahaj.zone is 1.3 million activities behind on lemmy.world for example, but we're not behind on any other instance, because those channels don't hit capacity. Now, if we could use multiple channels at once to talk to lemmy.world, we wouldn't have a problem, but lemmy isn't built for that at the moment
and you end up with a single channel that gets flooded.
The idea behind federation was that you don't get single instances so large that the single sync channel gets flooded. You're meant to have more smaller instances spread out that then federate with one another like a mesh- only one copy of data needs to be sent to each instance to serve additional thousands of users without further load.
That said, realistically the fediverse needs single instances with 10-30k users to really thrive, and L.W hasn't even hit 10k actives... so there does need to be some batch improvements in the backend still.
Having the ability to "merge content" from multiple instance communities would help a lot too. People congregate on L.W both from inside and outside the instance because it's big and has the most content. But if you have multiple communities (e.g, search "memes" and see how many different memes communities there are) the others with less content get ignored. Being able to merge those from a user viewing perspective, so you would just have a "memes" group that sees federated content from all the memes communities, could reduce the need for L.W to be all encompassing.
I think it's something where Kbin sends an insane amount of duplicate AP messages or something like that.
Every time my instance falls behind with them, there's usually some community ban in LW's modlog soon after about a kbin user sending suspicious traffic. My understanding is that's unintentional on that user's part.
I thought there was a post breaking down the problem but I can't seem to find it.
Edit: It was in admin chat, not a post (or if there was a post, I still can't find it).
Kbin (the software) is like the bad neighbor in the fedihood.
This seems to be way more impacting at this point. I'll check if there's anything I can do, but since it seems to be affecting other larger instances as, not sure how much that would be.