MP4 videos contain a flag that tells the player when it can start, called the moov atom. When this flag is set at the end of a file,
some players will wait until the entire video is downloaded before they start playing. To speed up this process,
always look for a checkbox called ‘fast start’ or ‘web optimized’ or ‘streaming enabled’ and enable it when exporting a video.
If you already have a video and want to make it start faster, do this:
Make a video start faster by moving the MOOV atom
This tells ffmpeg to simply copy the audio and video (no recompress) and only move the flag:
ffmpeg -i input.mp4 -c:a copy -c:v copy -movflags +faststart output.mp4
(Note: the input file must be listed before using a copy codec).