I am not very familiar with the automatic transmission on the more recent 3 series. Does it have different modes/settings? I would check to ensure you aren't running some setting that makes the car slower off the line.
Is it just slow? Or is it slower than other 328i wagons?
When you say it is slow compared to other cars, what other cars are you comparing it to?
Since it is CPO, I would take it to the dealership. If they claim acceleration is typical for 328i wagons, I would ask for a test drive on one or two off their lot
Edit: Your six-speed Steptronic automatic includes normal, sport, and manual shift modes and you should expect 0-60 time of 7.1 seconds.