The following command is to trim “INPUT.mp4” from start time 00:00:00 for a total play duration of 01:33:00, and save it as “OUTPUT.mp4“. No matter what the INPUT.mp4 file size is, this command is done almost immediately.

ffmpeg -ss 00:00:00 -i INPUT.mp4 -t 01:33:00 -vcodec copy -acodec copy OUTPUT.mp4