The rate of playback can be controlled in two ways. The Frame Skip controls change the step size. By default, the frame skip is 1, so each step of the playback increases (or decreases) the animation frame number by one. If the frame skip is 5 then the animation proceeds five times faster because only 1/5 of the frames are shown.
The Speed slider at the bottom of the form also affects the playback speed. Internally, this controls how many screen updates are needed between each step. By default, the slider is at the far right indicating that one step is performed for each screen redraw. Moving the slider to the left increases the minimum time required between updates.