Zurich airport is offering PCR covid test with result the same day.
But it's not possible to book the time, it's walk-in.
I asked, and they are not able to say best time to go or expected waiting time. They just said that's very irregular.

There is a "Fast track" option for an extra 50CHf, which I'd rather avoid...

Anyone has experience or info on the waiting time, or a suggestion on best time to go?

Early morning during the week. Definitely not on Saturday morning

I went there three times in total and can confirm that it is very irregular. I had everything from no waiting time at all to two hours. I recommend to go in the evenings - they open till 10, so after 8 is probably a lot better than just after work.