All 8 batteries fully discharged

#1

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?

#2

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