Do you mean that the slider indicating the current position in the buffer does not relate to the start and end sliders? How smooth things appear on screen is largely dependent on the guirefresh() rate. The lower this is the better the GUI responds, but the more CPU that is used up. At normal speed the instrument seems to move between the start and end points without any noticeable issues. But at higher speed it starts to look less synced. The audio will be fine however. No need to worry about that.
Finding a generic solution to the graphics issue is not easy. If one really needs to have a GUI control that updates very fast, it's best to write a new control in C++.