Frank Reiner: In my case the slowly means 5-10 seconds. Yesterday I also measured the CTS, the values are within specs. If I unplug it I get static 30% duty cycle which is correct.

nulu: The ignition coil was already replaced twice, pretty much the whole ignition system is new.

I am still suspecting some kind of electrical issue, because unplugging the O2 sensor should give static 50% duty cycle. But again pretty much all of the electrical parts are checked.