Still why didn't you did that:

    import scala.concurrent.duration.Duration
    import scala.concurrent.{Await, Future}
    import scala.concurrent.ExecutionContext.Implicits.global

    object FutureGen {

      def caller(num: Int, size: Int, div: Int): Future[Long] = {
        if (size == 1) {
          Future.successful(num.toLong)
        } else {
          val futures = for (i <- 0 until div) yield caller(num + i * (size / div), size / div, div)
          Future.sequence(futures).map(_.sum)
        }
      }

    }


    object Root extends App {
      // Warmup
      Await.result(FutureGen.caller(0, 1000000, 10), Duration.Inf)

      val startTime = System.currentTimeMillis()
      val x: Long = Await.result(FutureGen.caller(0, 1000000, 10), Duration.Inf)
      val diffMs = System.currentTimeMillis() - startTime

      println(s"Result: $x in $diffMs ms.")
    }

This would give Scala: 499ms and Go: 436ms which is pretty neat, since go actually has no type system. (And I'm using the global execution context, which could also be changed) Currently your akka version is slow, since it's creating a new class on every call, so you would have like a dozen more methods to dispatch than my version.