Test builder vs. update performance for HashMap

HashTest.scala

object HashTest extends App {
  import scala.collection.generic.CanBuildFrom
  import scala.collection.immutable.HashMap

  type HM = HashMap[Int, String]
  def updated(map: HM, key: Int, value: String) = {
    map.updated(key, value)
  }

  def builder(m: HM, key: Int, value: String)(implicit cbf: CanBuildFrom[HM, (Int, String), HM]) = {
    val builder = cbf(m)
    m.foreach { case (k, v) => k -> (if (k == key) value else v)}
    builder.result
  }

  val map = HashMap((0 to 10000).map(p => p -> p.toString): _*)
  def now = System.currentTimeMillis

  for (rep <- 0 to 10) {
    {
      val time = now
      for (i <- 0 to 10000) {
        updated(map, 500, "xxx")
      }
      println(s"Update ${now - time}")
    }

    {
      val time = now
      for (i <- 0 to 10000) {
        builder(map, 500, "xxx")
      }
      println(s"Builder ${now - time}")
    }
  }
}

Output on my system is:

Update 14
Builder 1924
Update 24
Builder 1758
Update 2
Builder 1729
Update 1
...