ffmpeg vs. build-in MKV demuxer

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(This is all with master HEAD build on OS X)

Because of some issues I've observed with the VLC mkv demuxer, I'd like to instead use the ffmpeg one. I deleted the MKV module, and sure enough the ffmpeg demuxer was enabled. Unfortunately, playback appears really jerky (sound is fine, but video plays in fits and starts).

Any thoughts?