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.
Recording a vote from an already-known account is way less than 100 ms, yes. Usually it's less than 100 ms but sometimes it can be several seconds.
When an activity is received the cryptographic signature on it needs to be checked and that means sending a network request to the creator's instance to retrieve the creator's public key (and profile pic, and cover pic each of which are more network requests. Then resize and store those images).
Images in posts need to be downloaded, resized, scanned for objectionable material, etc.
Every network request is quite unpredictable as many instances are overloaded or poorly configured.