SORT and STABLE-SORT accept two additional keyword arguments :START and :END:

(SORT sequence predicate &KEY :KEY :START :END)
(STABLE-SORT sequence predicate &KEY :KEY :START :END)

SORT and STABLE-SORT are identical. They implement the mergesort algorithm. Worst case complexity: O(n*log(n)) comparisons, where n is the LENGTH of the subsequence bounded by the :START and :END arguments.