[FFmpeg-user] Better way to cut parts out of a long video?

[Cecil Westerhof]

I know that because of iframes you need to put the inputfile before
the start and to parameters if you want to sure that the start time is
correctly used. Because of this I defined the following function.
    function doConvert {
        echo "Convert $1 to $2"
        time nice -n 19 ionice -c3  \
          ffmpeg -y -i $1           \
            -ss $3 -to $4           \
            -vcodec libx264 -crf 26 \
            -acodec libmp3lame      \
            -qscale:a 9             \
            -preset veryfast $2
    }

Normally this is not a problem: I do not have to cut from a video very
often and mostly the video is not very long. But this weekend I needed
to cut three parts out of a long video and this took more as 1½ hour
and my processor was (on my system) completly hogged:
    doConvert Original/00233-00239.MTS improvisatieVoorEnTegenDeTienGeboden.mkv 2658 2748
    doConvert Original/00233-00239.MTS improvisatieWelOfNietOntbijten.mkv       2757 2846
    doConvert Original/00233-00239.MTS improvisatieWatWeetJeOverIjsheiligen.mkv 4620 4682

The time output was (the first one I find a bit strange):
    real    45m33.66s
    user    63m9.35s
    sys     1m0.51s
    perc    140.83


    real    23m18.92s
    user    60m39.46s
    sys     0m52.01s
    perc    263.88

    real    29m36.04s
    user    88m38.41s
    sys     1m4.94s
    perc    303.11

This is because ffmpeg spends all its time seeking the start of the
video. In the last case that is more as 1¼ hour into the video.
I am not using ffmpeg that much so maybe I am asking something stupid.
Would it not be possible to skip to the last iframe before the start?
Or should I be doing things completly differently?

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof