10.5.4 Allocation of stacks
Stacks are allocated directly in generation 2 because they are relatively expensive to promote. Therefore creating many processes will cause generation 2 to grow, even if these processes are short-lived.
*default-stack-group-list-length* controls the number of stacks that are cached for reuse. Increase its value if your application repeatedly makes and discards more than 10 processes.
LispWorks User Guide and Reference Manual - 22 Dec 2009