Is it possible to adjust the frame rate of viewport playback? Is it possible to play an animation in the viewport at a slow framerate?

Just because of the rather heavy scene it's already going at about 8 fps, but I would like to be able to play it even slower (an in a controlled manner) for the purpose of recording an animation with automatic keyframes.
The Frame Rate in the render settings should also control viewport playback. If you choose the "custom" option and set it to 8, it should make the viewport also play at 8 fps.