What are lazy queues, anyway? Lazy queues are queues where the messages are automatically stored to disk. Only when they are needed are messages loaded into memory.
This differs from how default queues work, where messages are only paged out to disk when the memory is starting to get full. This paging process can take some time and will block the queue from processing messages. If there is a lot to page out, it can take considerable time and will effect the performance of the broker. With lazy queues the messages go straight to disk and thereby the RAM usage is minimized. A potential drawback with lazy queues is that the I/O to the disk will increase.
CloudAMQP has found the lazy queue feature so good that our new clusters (with RabbitMQ versions 3.6.0 and higher) comes with this policy enabled from the start.
If you are running a 3.6.x cluster without lazy queues you can enable it by adding a policy for it.