Selfhosted

I had a few ideas, I’m suspicious that handbrake is falling back to CPU, maybe check the logs of the container to make sure it isn’t falling back to CPU decoding. Otherwise here are a few things I would check next:

• If you are not using docker locally so you are already doing this, you will need to configure the docker container to pass through the GPU for quicksync to work inside the container.
• If you are already doing that then I would make sure the device is the same name on the synology, it probably is but just to be sure.
• you will likely need to add your user to the video and/or render group on the synology if you haven’t, especially if you are running the container as your user instead of root
• make sure you are reading and writing to volumes that use bind mounts and not docker volumes, overlayfs is not what I would call fast and writing especially.

If all the settings and versions match, my first suspicion would be that there's some library available on one system that gets used but not the other. Have you taken very verbose logs to see if there's differences that stand out?

IIRC, presets are machine dependent, or at least they are optimized for the hardware. So if you have a weak machine, veryfast will use even worse encoding because you're simply saying "I just want it done fast", while on a more robust machine it may take more liberties. To match it completely I think you need to disregard the presets and set everything yourself, like CRF and everything.

Why are you encoding a file that’s already been encoded?