High speed mode will be enabled when the image pattern is raw8. In this mode, we will use a lower bit of AD conversion than the normal mode, (most of the time, it is still bigger then 8 bit), And remove the extra bits and upload the data.
So this is the reason why there is no effect on the image.
For the speed, for most of the sensor, a lower bit of AD conversion will have a higher framerate, for some case, a higher framerate will be more useful.
Another information: In the past, the data rate of the USB bandwidth will be larger than the image sensor's data rate. In this situation, the framerate of the camera is limited by the sensor. And highspeed with Raw16 could not get a better performance, frame rate increases are limited, while image quality is greatly reduced. That's why we do not support the Highspeed with Raw16.
But now, things are different, image sensor has a larger data rate than USB. So in the future, we will support the Highspeed mode with Raw16.