My input would be :
**** Extremely rapid ****
(just to add to "darned fast" and "almost too quick"). Sorry OP, know these hyperbolic phrases don't really help, but even I would love to get this info from some reliable source.
However, here is some related info:
The F30 328i does the 40-70 in 4.5 s as per motortrend
The E90 with N54 motor did a 0-60 in 5.1s (which is close to the F30 N55) and a 40-70 in 3.9s (source
). I would take a guess that the new 335i does a 40-70 very close to that figure (somewhere between 3.8 to 4 seconds)