Mouse trailing

When I drag GUI elements with my mouse they trail after it by one frame, they don't move instantly after it. Tell me please how to fix it.

I tried to use the standard GUI.Window element, I even tried to write my own, counting mouseDelta like this:

     void Update ()
     {
             MouseDeltaX = Input.mousePosition.x - OldMouseX;
             MouseDeltaY = Input.mousePosition.y - OldMouseY;
             
             if(Input.GetMouseButtonDown(0))
             {
                     isLeftButtonDown = true;
             }
             if(Input.GetMouseButtonUp(0))
             {
                     isLeftButtonDown = false;
             }
             
             if(isLeftButtonDown)
             {
                     StatsWindow.SetPos(StatsWindow.xpos + MouseDeltaX, StatsWindow.ypos - MouseDeltaY);
             }
     
             OldMouseX=Input.mousePosition.x;
             OldMouseY=Input.mousePosition.y;
     }
     
     void OnGUI()
     {
         StatsWindow.Display();
     }