Parallel sending of federated activities to other instances. This can be especially useful for instances on the other side of the world, where latency introduces serious bottlenecks when only sending one activity at a time. A few instances have already been using intermediate software to batch activities together, which is not standard ActivityPub behavior, but it allows them to eliminate most of the delays introduced by latency. This mostly affects instances in Australia and New Zealand, but we’ve also seen federation delays with instances in US from time to time. This will likely not be enabled immediately after the upgrade, but we’re planning to enable this shortly after.
On the other hand, it's probably easier to decentralize while we have 55K MAU rather than 500K
That's also true, can't argue with that.