When you spin the FC knob, it will queue up tuning commands and enable acceleration. The acceleration is not linear due to the shaft encoder used in the FC, but it is close.
Also it looks like that the spectrum display repaints after the slice has changed frequency is probably the reason for the lag. As others have noted, with an adequate video card that is able to do fast rendering, the display anomaly is not seen. You may want to consider a video card update.