Concurrency in recursive function

Question

I have function like this

def jsonFlatten(obj: JsValue, coun: JsObject = Json.obj(), pref: String = ""): JsObject = {
var con = coun
var kel = ""
var el: JsValue = Json.obj()

obj match {
  case JsObject(_) =>
    var ob = obj.validate[JsObject].get
    ob.value.foreach({
      case (k, v) =>
        if (pref != "") {
          kel = pref + "." + k
        } else {
          kel = k
        }
        el = v
        el match {
          case JsObject(_) =>
            con = jsonFlatten(el, con.++(con), kel)
          case JsArray(_) =>
            con = jsonFlatten(el, con.++(con), kel)
          case _ =>
            con = con.+(kel -> el)
        }
    })
  case _ =>
 }
return con
}

That function called on another iteration within thread and I want to optimize this function because it takes lot time (around 300ms) and I have to call lot of that function in iteration.

Is there any suggestion how to optimize that function with concurrency or effective recursion ?

Thanks as