The downlink speed has little to do with the smoothness. Rather it affects the required bandwidth. Increasing the bandwidth, increases the display resolution, but reduces the range of the signal. During a flight, I would submit you do not necessarily need great resolution, to determine where the camera is pointed. The images recorded locally on flash, are at the selected resolution. I would tend to reduce the display resolution to increase the reliability of the signal.
If you are feeding live video somewhere, that may not be the case. But most of us are recording video (or photos) for viewing/editing later