While rotating the board using the cursor keys, to be exact while one of the cursor keys is pressed down, some of the above mentioned display improvements are temporary disabled to speed up the rotating. While rotating the whole scene has to be redrawn quite often, so every unnecessary feature will slow this down. And while the picture is moving, not the best quality is required.
Once you release the key, the final scene will be drawn with all settings enabled again. This should make rotating more efficient while the quality drop should not be very noticeable during the movement. If you have a fast computer, you can probably keep this disabled. Try and see if this makes a significant performance difference. Once you stop rotating or moving the board, this option does nothing.