Most people don’t realize that the frame rate is adjustable, for Slo-motion, on the iPhone 5S, 6, and 6 Plus. You’ve got 2 settings: 240 and 120.  The image below shows this iPhone 6 Plus using 240 (upper right hand corner).  Click on it to alternate between the 2 settings.  The higher the frame rate the better the slo-motion will look BUT the larger the file will be.  Use 120 if you’re tight on space and not concerned with high quality.  Use 240 if you’re looking for a smooth and high quality slo motion recording AND have the room to store it.