Fairly quick process at the counter that is inside the terminal building, but the keys had to be collected from another Sixt facility closer to the garage where the car was. So there was some unnecessary waiting time as all I needed was the key, and there were other customers ahead of me who needed to undergo the full rental process.
But once it was our turn, we were given the keys right away.
And, they did not have the exact car I had rented so we got a better equipped model instead.
Overall a really good experience.