[FFmpeg-user] Slow speed when sub-clipping .mxf

[Erik Dobberkau]

Options generally apply to what follows, not what precedes. Putting the
> seek time before the input spec seeks on the input, putting it after seeks
> on the decoded stream.
>

Which is important when using pre-input seek with GOP codecs, as it will
„snap“ to the next (IIRC) keyframe (I-frame), contrary to intra-frame-only
codecs, where the correct seek time will also start reading exactly from
the intended frame / time. YMMV.