When the "Toggle div" button is pressed, we toggle between hiding and showing the div (default is 13 milliseconds). Each time the "Run animation with less frames" button is pressed, we add 500 milliseconds to the property, which causes the animation to run with less frames (depending on the browser, this may cause the animation to run less smoothly than desired).
Note: Since jQuery uses one global interval, for any changes to this property to take effect, no animation should be running or all animations should be stopped first (press "Toggle div". Then press "Run animation with less frames" when "div" is finished with the animation. When the animation has stopped, press "Toggle div" again to notice the effect).