Oli,
looks like you are alwys in the forum, really interesting
The reason for this should be the static pressure, the air is not coming in straight and get blocked a bit by the dust filter. If you want a better result you might need to replace the fan, but I'd say it is ok as it is if you just remove the dust filter.
The duct project is interesting, I hope you can still make something.
Greets,
Andy