If you don't already have 4ga cables, that would be the first place to start. That would give you more available torque for hills but will do nothing for flat ground speed.
Going faster on flat ground would require a motor change, gear change or tire size increase.
One solution that would cover both would be to do a 48v conversion. The cost to do this can be around $1500 for new or additional batteries, controller, solenoid and charger.