Queue delay not working as intended

I have a code which makes api call for list of items to fetch data of , then it pulls data of each item. Let's say it looks like following:

var Queue = require('bluebird-queue');
api.getContainer(contaierId)
  .then(function (items) {
    var itemQueue = new Queue({
      concurrency: 1,
      delay: 2000
    });
    itemQueue.add(items.map(function (item) {
      return api.getItemDetails(item); // returns promise
    }))
    return itemQueue.start();
  })
  .then(function (itemDetails) {
    ...
  });

getContainer and getItemDetails are functions which return Promise, which return request.
After I start the queue, first getItemDetails is called properly after a delay, but then it invokes everything else and waits for the remaining time.

bluebird: v3.4.5
bluebird-queue: v0.0.8
node: v4.4.7

Edit: Posted more accurate example.