How to … Upload Slow-Mo Videos to YouTube
Oftentimes, and particularly for better players, slow-mo videos are the only way to really see intricate parts of the golf swing needed to diagnose an issue properly.
Record your video in Slow Motion on your iPhone. Edit it to trim the slow-mo start and end points as well as to trim off any extra "footage" you don't need.
Open the iMovie app on your phone and make a new video with your swing clips. If you have multiple clips you want to put in one video, import all of them.
Export the video to your camera roll. For example, a 0:04 video will be about 0:19 when done this way.
Upload the new video, with the slow motion "baked in," to YouTube or Vimeo or whatever.
You can also record your screen playing the video back in slow-mo, but this limits the video resolution to that of your display, not the original, most likely larger resolution of the video file itself.
Make sure your video is unlisted or public, as private videos won't work.
This works for iOS/iPadOS 14. Small modifications may be needed for earlier or later versions of iOS and iPadOS.