Might be some kind of linked list like implementation under the hood. Getting the first element is performant but traversing everything to find the last one would be slow.
Even for singly linked lists, implementations for general purpose use usually keep pointers to the first and last elements so that you can use it as an O(1) FIFO queue, as well as prepend in O(1) time.
If you only keep a pointer to the head of the linked list, then it can be practically used a LIFO queue (stack), and a few other corner cases, but not much else. (A FIFO isn't very useful if adding an item is O(1) but removing an item is O(N), or vice versa.)
