No, the ball will get there faster, but it will still be 70 mph. We are not measuring say... "time to the plate" we are measuring the miles that ball would go if it traveled at that constant speed for one hour.
Say one kid was about to throw on a LL field and another on a Majors. Suddenly, as they release, the backstop and the catcher are gone, and the ball keeps going.
70 mph would be the same both ways.