If the CPU or HD is busy and I press "next" (down arrow) to move to the next picture, it will still accept the keyboard command but not load the picture or move down the list, then when it comes back it will skip several pictures. It should not move on till it actually shows the next picture.
It should wait for at least 75% of the picture to load before moving to the next picture, if the keyboard repeat is too fast.
The updating of the list should occur only when there is time available, meaning it should not be a priority to update the list, but showing a pic should be.
The use in this situation is security pictures, there may be over 5000 in one folder and you need to move through them quickly, or your life will be wasted :)