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 mp: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 - 13 Feb 2015