Why my 4WD Bluetooth car run so slow
I can't find issue in your code that I'm convinced is your problem. Without seeing your datasheet for the motor, I would bet your issue is one of two things:

1) A 9V battery is low-current relative to most batteries I've used for powering any decent DC motors. Do you have access to a better current source you could try?

2) Every motor has a trade-off in torque vs max-speed, dictated by the internal gear-ratio. Depending on the motor type, it may not have the torque to accelerate the vehicle to a significant speed.