From what I understand DSG and hence M-DCT downshifts CAN be slow if the system has not correctly anticipated the need for a lower gear. If it has, which it can do most often, the downshifts will be precisely the same speed as the upshifts. VW quotes 8 ms for DSG but a more realistic number from other sources is about .02-.03 s. This time is also likely the ENTIRE shift with both clutch operations. As we know SMG quotes times of .065 or .08 s are BS, including both clutch operations the actual time is closer to .20-.25 s
DSG and hence again M-DCT is inherently smooth shifting. I doubt they will provide a mode to cripple the system by making it more like SMG. I agree there is some raw appeal of the neck snap of SMG but fast=smooth=more performance=less wear with DSG/M-DCT. Along with the entire nature of the new M3 we are going to have to acccept that more raw/visceral is not faster nor better in many cases.