That bench was wrong. Here is a correct:

    th.pbenchOff() {
      var b = mutable.ArrayBuffer[AnyRef](); (0 to 10000).foreach { b += _.toString }
      while (b.nonEmpty) b = b.drop(5) 
    } { 
      val q = mutable.Queue[AnyRef](); (0 to 10000).foreach { i => q.enqueue(i.toString) }
      while (q.nonEmpty) { q.head; q.dequeue }
    }
    Benchmark comparison (in 14.87 s)
    Significantly different (p ~= 0)
      Time ratio:    0.01729   95% CI 0.01698 - 0.01759   (n=20)
        First     56.43 ms   95% CI 55.95 ms - 56.90 ms
        Second    975.4 us   95% CI 960.5 us - 990.3 us