Slowing down footage with easing (like iPhone 5s)
You can do it by adding transitions between the speed changed clips as described in the documentation section titled Add Transitions Between Speed Segments. They say:


  When two adjacent speed segments are set to play at different speeds, you can control how smoothly the video switches between the playback speeds. When there’s no transition between the segments, playback shifts suddenly from one speed to another. By adding a transition between the segments, you can create a more gradual, or ramped, effect. You can also control how quickly that transition takes place.