The standard speed is typically around 28 frames per second. If you set your playback speed VERY LOW....like at 15%, you are taking the same number of frames as the original, and slowing them down so that each frame will appear on screen for a longer period of time.
This can give the appearance of 'Jerkyness' as each frame shows on screen many times longer than the normal split-second view.
If this appearance bothers you, you can speed up the playback. It's not that noticeable at midrange speeds (like 50%), but gets really noticeable if you drop much lower.
SOLUTION: Increase your playback speed