When we write out a file we use the selected frame rate. For example, if you have selected 120fps, a file will be created with 120fps. Within MAVIS (and other apps) when you play back a file the default playback speed is the same as the capture speed therefore it will be played back at 120fps and it will look normal. This allows you to take the video file and use it within a edit package with maximum flexibility.
However, if you save a 120fps file to the Photos library, iOS will automatically recognise that it is a high frame rate video and will play back the file at a slower rate (e.g 30fps), thereby giving you a slow motion video.