FFmpeg - MP4 Faststart

Normally, a MOV/MP4 file has all packet metadata stored in one location. This data is usually written at the end of the file, but it can be moved to the start for better playback by adding +faststart to the -movflags. This allows downloaded files to begin playback immediately.

ffmpeg -i <input.file> -c copy -f mp4 -movflags +faststart <output.file>

Depending on the size of your input it can take a few more seconds usually to perform the second pass to move the moov atom to the beginning of the file.
This will… allow the video to begin playing before it is completely downloaded by the viewer.
Reference: [FFmpeg: FFmpeg Formats Documentation]
Reference: [superuser: Any downsides to always using the -movflags faststart parameter?]
Reference: [FFmpeg: H.264 Video Encoding Guide]