First. You have to understand batteries dont mean speed. Batteries = run time. Bigger batteries mean more range.
Speed is determined by current and voltage. Higher current and voltage creates more speed. Speed in an electric cart gets expensive quickly. Upgrading motor will give more speed immediately. But to maximize the potential of the motor you need a high current controller, solenoid, FNR switch, and cables.
You could start with cables, old cables and high resistance will hurt performance. But be warned, upgrades are addictive. If they weren't this forum wouldnt exist
My DS will cruise 22-23mph on stock 18" tires. But it's got around $2k worth of upgrades on it. Lift kit and bigger tires are in my future if I decide to keep this one around for a while:)
Bigger tires may help you a bit too, but will hurt acceleration a bit but you'll gain top end speed