When you play a scene, you see a rough approximation of the animation. The playback speed and quality is imprecise as compared to rendering each frame and playing the frames in sequence with a playback utility specifically designed for this purpose. The reason for this is that Maya processes the animation of each frame before display, while a playback utility simply plays previously processed and rendered images.
The more detail that Maya needs to process, the slower the playback. For example, playback will likely be slow for any scene that has several complex objects displayed with Smooth Shade All and Hardware Texturing.