Sending and consuming 5000 messages to/from a queue, one can see heavy CPU use on the broker side (v 5.12.2).
as a hot spot method. It calls ArrayList.add(Object) around 12 mio times.
Situation is that FilePendingMessageCursor.isEmpty() iterates over in-memory messages and therefore (as it is a prioritized queue) uses PrioritizedPendingListIterator which uses OrderedPendingList.getAsList() which overall turns out to be an expensive method as it converts the self-managed linked list to a Java ArrayList and then this list is filled into another ArrayList managed by PrioritizedPendingListIterator.
PrioritizedPendingListIterator could be improved to walk the priority lists via OrderedPendingList iterators, as these are implemented efficiently.