[JimD]

It's probably mostly handling but the drag coefficient is worst on the 135, it may make some difference. The BMWUSA does not seem to list the 135 couple coefficient any more but the convertible is .34 versus .30 for the 335 couple. The weight of a 135 couple is only 187 lbs less than a 335 couple (both with manual transmissions). So even though the 135 is smaller, it doesn't weight a lot less and it has a higher coefficient of drag.

.4 seconds out of 2 minutes does not seem like a huge difference to me. But I would also think with the same power a smaller car would be faster. And it is rated quicker 0-60 (5.1 versus 5.3 seconds).