Siphon - Flush water flows extremely slowly after replacement

This is Lillviken from Ikea. Unfortunately, they have their own system, so no siphon from the hardware store would fit. So either replace the sink and siphon or find a solution

Is the 90° bend possibly inserted too far into the wall/drain?

Or is your sewer pipe not vented?

Is the 90° elbow possibly inserted too far into the wall/drain?

New idea what it could be, but unfortunately no.

Or is your wastewater pipe not vented?

I can't judge as a layman, but previously the predecessor model of Lillviken was installed without additional venting and with that the water always drained smoothly.

Thank you very much for your efforts.

Dismantle and hire a professional

Dismantle and hire a professional

Maybe someone else has ideas that you wouldn't think of yourself.