It's not the fastest often because it's oversubscribed and people do not understand that the car has a 3rd, mostly underuntilized, state of neither pedal depressed (ie "coasting") ... so they create cascading braking pileups ...
I much prefer cars with short gearing for better engine braking. I continue to choose to drive a manual party for better engine braking.
If they'd just drive 70 on cruise control, they'd be faster, less traffic jams and less fuel use...
I really would like to see some mandatory use of cruise control on some stretches of highway, even if just as an experiment