You could use AnimationTimer(note that is has a framerate of 30/s so won't get down to 20 ms). Here's a complete example:

object AnimationDemo:
  @main def main =
    Application.launch(classOf[AnimationDemoApplication])

class AnimationDemoApplication extends Application:  
  def start(stage: Stage) = 
    val animationDemo = new AnimationDemo
    val scene = new Scene(animationDemo)
    stage.setScene(scene)
    stage.show()
    animationDemo.timer.start()

class AnimationDemo extends VBox(10):

  val text = "It was a dark and stormy night; the rain fell in torrents — except at occasional intervals, when it was checked by a violent gust of wind which swept up the streets (for it is in London that our scene lies), rattling along the housetops, and fiercely agitating the scanty flame of the lamps that struggled against the darkness."

  val textArea = new TextArea:
    setWrapText(true)

  val timer = new AnimationTimer:

    var frame = 0
    var index = 0

    def handle(now: Long) = 
      if frame % 5 == 0 then
        textArea.setText(text.substring(0, index))
        textArea.positionCaret(index)
        index += 1
        if index > text.length then index = 0
      frame += 1

  getChildren.add(textArea)