为什么Scala中的相同算法比C#慢得多？以及如何让它更快？

C#代码：

static void Main(string[] args)
{
  var arg = new List<List<int>>();
  int i = 0;
  for (int j = 0; j < 5; j++)
  {
    arg.Add(new List<int>());
    for (int j1 = i; j1 < i + 3; j1++)
    {
      //if (j1 != 5)
      arg[j].Add(j1);
    }
    i += 3;
  }
  List<Utils<int>.Variant<int>> b2 = new List<Utils<int>.Variant<int>>();
  //int[][] bN;

  var s = System.Diagnostics.Stopwatch.StartNew();
  //for(int j = 0; j < 10;j++)
    b2 = Utils<int>.Produce2(arg);
  s.Stop();
  Console.WriteLine(s.ElapsedMilliseconds);

}


public class Variant<T>
{
  public T element;
  public Variant<T> previous;
}


public static List<Variant<T>> Produce2(List<List<T>> input)
{
  var ret = new List<Variant<T>>();
  foreach (var form in input)
  {
    var newRet = new List<Variant<T>>(ret.Count * form.Count);
    foreach (var el in form)
    {
      if (ret.Count == 0)
      {
        newRet.Add(new Variant<T>{ element = el,previous = null });
      }
      else
      {
        foreach (var variant in ret)
        {
          var buf = new Variant<T> { previous = variant,element = el };
          newRet.Add(buf);
        }
      }
    }
    ret = newRet;
  }
  return ret;
}

Scala代码：

object test {
def main() {
 var arg = new Array[Array[Int]](5)
 var i = 0
 var init = 0
 while (i<5)
 {
  var buf = new Array[Int](3)
  var j = 0
  while (j<3)
  {
   buf(j) = init
   init = init+1
   j = j + 1
  }
  arg(i)=buf
  i = i + 1
 }
 println("Hello,world!")
 val start = System.currentTimeMillis
 var res = Produce(arg)
 val stop = System.currentTimeMillis
 println(stop-start)
 /*for(list <- res)
  {
   for(el <- list)
    print(el+" ")
   println
  }*/
 println(res.length)
}

 def Produce[T](input:Array[Array[T]]):Array[Variant[T]]=
  {
   var ret = new Array[Variant[T]](1)
   for(val forms <- input)
   {
    if(forms!=null)
    {
     var newRet = new Array[Variant[T]](forms.length*ret.length)
     if(ret.length>0)
     {
      for(val prev <-ret)
       if(prev!=null)
       for(val el <-forms)
       {
        newRet = newRet:+new Variant[T](el,prev)
       }
     }
     else
     {
      for(val el <- forms)
        {
         newRet = newRet:+new Variant[T](el,null)
        }
     }
     ret = newRet
    }
   }
   return ret
  }


}

class Variant[T](var element:T,previous:Variant[T])
{
}