Bad UI response when the rendering points is too big

I am using winform version. If the points is about 1M points, When I try to RectangleZoom the rendered  model, the ui performances is relatively too worse for users.When I drag the model, the rectangle barely shows in a  few seconds.The zoom rectangle should be shown in a  much faster speed.How to improve the  response  speed, when the points to  render is  bigger than 1M points.