Pierre: just some commentary here.

I wish it had worked better....

That is what I was afraid of.

I did a lot of tests with the same.xml file that contains 4 mixers. Each
time I checked both with mpeg proxies and original DNxHD sources.

When I load DNxHD sources, it falls back to using software as seen on the startup window:

Decoder dnxhd does not support device type vdpau.
HW device init failed, using SW decode.
file:/root/Downloads/DNx145_1080i_2997.gxf
err: Success

FFmpeg apparently can not handle dnxhd with vdpau so that would be why you see not speed up for those.