Batched UPDATE processing
There's a cool thing that can happen when you have a queue whose consumer can work in batches: as the load increases, the batch size increases, so the per-batch overhead is amortized over more queue entries and the system's overall efficiency goes up.
This occurs in the DNS for UPDATE processing: UPDATEs are serialized, so new UPDATEs must be queued while an UPDATE is in progress. It then makes sense when an UPDATE is completed to process all pending UPDATEs in a single transaction, so the server has a chance of catching up with its backlog.