My kitchen sink drains very slowly. From full it takes more than a minute to fully drain, and it is hard to perceive the water level dropping.
Listening in the cabinet underneath I do hear the trickling of water. I removed the S-trap to clean it, and to see whether it was blocked, but there was no significant debris on or in it, just some slime on the side.
After putting it back, I tried whether opening the aerator cap (at least, that's what I think it is, marked in red on the picture below) on the S-trap helped with the draining. It did improve the draining speed significantly, but water also leaked through the cap, so clearly it's not intended to be opened.
What else can I do to improve drainage speed?