If one wants to append a IntArrayList to another IntArrayList, there's an extra copy involved due to the call to toArray():

   public boolean addAll(IntIterable source)
    {
        return this.addAll(source.toArray());
    }

Also, please relax the visibility of items to protected, so one can subclass this collection. As I understand, that was the design rationale for FastList vs. JDK ArrayList, but looks like the same mistake is repeated here :)