Well i was just wondering how much does the 340 ohm resistor decrease it?
What is "it" ? Voltage, current etc?
You wont know the answer to either unless you know the resistance or current draw of the motor on its own.
eg if you just connect the motor to the supply, with no other resistors, then how much current does it draw - you'll need a meter.
In practice its more complex...If you apply a load to the motor shaft (try grabbing it in your hand) then you'll see it needs more current. Since V is fixed at 9V then if the current goes up then ohms law indicates that the resistance of the motor is not constant.
You are probably just running the motor with no 'stress' and you will no doubt find that the resistance of the motor is quite small. Hence adding an equivalently 'huge' resistor means that the voltage drop across the resistor is quite big - hence starving the motor of voltage so it runs slowly.
Before others answer then "Yes - this is a very basic answer" to a basic question.