Speeding up Arduino DC Motors
The speed of a motor is a function of the motor itself and the voltage applied to it. There is no programmatic way of increasing the speed of a motor. You would either need to increase the voltage being applied to the motor (as long as it is within the limits of your motor and speed controller) or find a different motor to use that is faster.

It appears the Adafruit motor shield accepts between 5-12 volts, if you are not already at the maximum of 12 volts, and your motor is rated to allow it, you can increase the voltage you are applying to the shield. All that 255 means in this case is give the motor all the power that is available, so if you are only giving the controller 5 volts, it is only going to give the motor 5 volts.

Also I might recommend instead of using analogWrite() that you use the official Adafruit library to control your shield as it uses its own PWM library which is more performant for motor control than the built-in Arduino one.

EDIT

Actually, with v2 of the shield, it has its own PWM driver controlled over I2C. In which case analogWrite() shouldn't work at all, or at least it doesn't do what you think it does. Instead use the official Adafruit library and I also recommend following the official tutorial as well.