The storefront can be perfectly fast while the queues silently back up — orders confirmed, but emails arriving an hour late, bulk operations stuck on "processing", andinventory_reservationrows growing because the cleanup consumer never caught up. This guide is about the layer most performance audits skip: the broker and the consumers that drain it. How Magento uses RabbitMQ Magento defines queues through three XML files per module:queue_topology. Messages published under a topic land in the broker, and a consumer process picks them up, calls the handler, and acknowledges the message. messages_unacknowledgedhigh: consumers are holding messages mid-processing, crashed mid-message, or stuck in a redelivery loop.