The aircraft can fly slower than the minimum drag speed. What this means is that it is hard work to accelerate towards this minimum drag speed. As the speed builds, the acceleration increases slightly. Above this speed, increasing drag through extra speed will reduce acceleration eventually to zero. Sometimes when the aeroplane climbs to its absolute maximum, it can be very difficult with maimum power to even speed up at all. Once the aeroplane does start speeding up, the attitude lowers slightly to its minimum drag figure and then speed can build quite rapidly. It is a normal effect. I don't see where wing flexing comes in at all or understand the turbulence quotation- I am sure it will be just like the 747.