Before revision: ffmpeg -i sample_input.mp4 -movflags faststart -acodec copy -vcodec copy sample_output.mp4 -movflags faststart does the optimization by moving the index (moov atom) to the beginning of the URL
After revision: ffmpeg -i sample_input.mp4 -movflags faststart -acodec copy -vcodec copy sample_output.mp4 -movflags faststart does the optimization by moving the index (moov atom) to the beginning of the file.