I have a new-to-me 2015 S4 with 30k miles. I noticed that on a cold start, it seems like the car is slow to fire up. I've never had a start failure but it seems like it is cranking too long. Subsequent warm starts seem to take a normal amount of time. I've also noticed that the engine idle can cause the car to shake a little bit, especially after the car is fully warmed up. It isn't super bad but definitely noticeable when parked or stopped, I can feel it through the seat. I had the car into the dealer for something unrelated and asked them to look into it, they said I had a very slight EVAP leak, which they fixed. I never had a CEL and honestly I didn't see any difference in the start up time or the idle after they fixed the EVAP.

Is this common with the 3.0T or is it worth taking back to the dealer?