Test.scala

object Test extends App {
  val benchIterations  = 10
  val warmupIterations = 10
  val repeat           = 100000

  val a = "foo"
  val b = 123

  def benchOne[U](a : => U): Long =
    (0 to repeat).foldLeft(0L) { case (total, _) =>
      val time = System.currentTimeMillis()
      a
      total + System.currentTimeMillis() - time
    }

  def bench[U](a: => U): Double =
    (0 to benchIterations).map(_ => benchOne(a)).sum / benchIterations.toDouble

  def warm[U](a: => U): Unit =
    (0 to warmupIterations).foreach { _ => bench(a)}

  def run[U](label: String, a: => U): Unit = {
    warm(a)
    print(s"$label: ")
    print(bench(a + b.toString))
    println(s" ms / ${repeat} ops")
  }

  run("Concatenation", a + b)
  run("Interpolation", s"$a$b")

}

Scala 2.11.8:

Concatenation: 13.6 ms / 100000 ops
Interpolation: 23.3 ms / 100000 ops

Scala 2.12.8:

Concatenation: 16.2 ms / 100000 ops
Interpolation: 14.9 ms / 100000 ops