Here are the specifications for HLS Authoring.
The Apple developer pages on iOS settings are the best references, but in general, following the above settings in your encoder will get you smooth, high quality video on iOS devices.
If you want extremely low bitrate video, you need to send 10 fps only, with keyframe every 30 frames, if the video stream is under 200kbps. For 200 to 300kbps, you want 15 fps and keyframe every 45 frames.