Does the radxa distro have the same performance issue or does it work as expected there?
You can download a image here https://github.com/radxa/debos-radxa/releases/tag/20230201-0944
My guess is that armbian is not using the correct GPU driver, and that results in CPU decoding.
The device tree in the radxa is different than the one in the armbian distro but I have no idea how to fix it.