Is it safe to assume if a battery pack had been fully discharged, and then the process of having to charge the batteries one-at-a-time to bring them back to a point the on board charger would work, and, they did charge back up to 100%, that this is the reason the car will only do 13-15 mph now as opposed to the 25-27 mph it used to go? Could there be any other reason its slower other than the batteries got ruined?
Charge them all up check the voyage of each battery individually with a volt meter. A fully charged 8v battery will read just under 8.5 volts. You might have a dead battery or two in the pack