Brief Intro

Queue Optimized - dequeue is not O(1)

More Details

 if array.count > 50 && percentage > 0.25 {
 array.removeFirst(head)
 head = 0
 }

If the condition happens, the complexity will be O(n/4) ~ O(n)
I think you should use LinkedList to implement a Queue to archive O(1) in all cases of dequeue