Condition variables implement a FIFO queue for waiting threads.  The current implementation uses a regular Python list but could use a deque instead.

A wait() call appends a new waiter.   A notify() call removes the oldest waiter; this is an O(n) operation on list but only an O(1) operation on deques.  A notify_all() call is O(n**2) for a list but only O(n) for a deque.

If there is interest in this patch, I can add slicing support to collections.deque so that this patch won't need itertools.islice()