Access to hash tables and non-simple arrays can be improved where they are known to be accessed in a single thread context. That is, only one thread at the same time accesses them.
that the table is going to be used only in single thread context, and therefore does not need to be thread-safe. Such a table allows faster access.
creates an array that is single threaded. Currently, the main effect of
is on the speed of
on non-simple vectors. These operations are much faster on "single threaded" vectors, typically more than twice as fast as "multi-threaded" vectors.
You can also make an array be "single-threaded" with set-array-single-thread-p.
The result of parallel access to a "single-threaded" vector is unpredictable.
LispWorks User Guide and Reference Manual - 21 Dec 2011