I think it's more like the previous commentor said. It's the communities more than the users. Every post, comment, like needs to be sent to every other instance that subscribes to the community. I suspect it's definitely connected to federation. The reason being, at 20:00 utc yesterday lemmy.world stopped sending my instance anything (previously it was between 2 and 5 messages a second). It only started again at around 00:00 utc. I wonder if they were slowly adding instances back to federation?

In any case the load for that many communities with that many other instances must be huge. The advantages of the fediverse requires that communities AND users are spread between instances. In the current climate, the super instances have most of both and it must be becoming exponentially harder to keep up with hardware requirements for this.