From the BMW site:
Time to 60 - 335i 5.1s, 335ix - 4.7s - the advantage of the xDrive is much larger for the 335, so I am not sure why you're bringing up the 335.
The lesson I'm getting from this is that four-wheel drive is faster until one is going pretty darn fast, and should have better traction and handling in a range of conditions.
The slightly less optimal weight distribution may be a downside, the car is 5mm higher, and the extra gears cost ever so slightly in gas mileage.
While I have little enthusiast driving knowledge, I do have a doctorate from Caltech, so I know some physics.