To clear things up: After I switched from Ikea to Standard, the problem still existed. But now I could experiment much better because it is simply expandable.
And lo and behold - with a drain pipe with an integrated air admittance valve (12€), the water drains faster (full basin) than you can count to 5.
I don’t have an explanation for that because the old siphon didn’t have an air admittance valve, but the logic behind it doesn’t matter to me anymore. The main thing is that the wastewater flows again.