The type unlocked-queue is a fast queue (first in, first out) that is unlocked, not thread-safe and does not have waiting functionality. It does not do anything that mailbox cannot do, but it is faster. It is useful when you always access the queue together with other operations that need to be "atomic", so that you need a lock around them anyway, or when queueing and de-queueing is done on the same process.
LispWorks User Guide and Reference Manual - 20 Sep 2017