[bambax]

Yes, copying the raw stream is the way to go if you're not rescaling, etc. This is the command to extract 15 seconds of video starting at the first minute, and not reencoding; it should be quite fast and it's also quite self-explanatory:

    ffmpeg -ss 00:01:00 -i in.mp4 -t 00:00:15 -c copy out.mp4

I'm really no expert so I just keep a few of those commands around... You don't need a deep understanding of video streams, etc. just to use ffmpeg.