You need to make an engineering judgment in your use case to accept the slower speed or adjust the poly line to approximate the spirals that the vehicle would follow as it transitions into the corners. I often see swept paths of vehicles following lines which claim to have been done at speed but in fact the user has opted to allow the wheels to turn on the spot thus removing the speed spiral all together, i believe that vehicle trackings process of attempting to drive at the requested speed and report on the speed needed to exactly follow the line is more honest. At the end of the calculation it reports the slower speed it needed to drive along the whole line. Vehicle Tracking will start with the speed selected in the model and try to follow the line, if any part of the turn can not be exactly carried out it will slow the vehicle down and try with the resulting tighter spiral. The issue is your polyline will be made up of arc's tangentially joining other arcs or straight sections, where as the vehicle turns through a spiral based on its speed and lock to lock time. The reduced speed is all down to vehicle tracking making the controlling point on the chassis (Usually the center of the front axle) exactly follow the picked polyline.
