Rendering video limited to 4 CPU cores?

Exporting a slideshow to AppleTV 1080p format on a Mac Pro Hexacore I see a process fmexport that uses about 400% CPU (in Activity Monitor). The machine is 50% idle.

Do you have a limitation to only utilize 4 cores on video compression?

The video compression process depends on the chosen video codec: Because the video codecs are provided by Apple (or other 3rd party) we don’t have influence on how many cores they are using for compression. We just provide the single video frames to the compressor and let QuickTime do it thing.