The lock stiffness is most likely not in the lock cylinder, but in the door latch mechanism. If you have power locks, the linkage to the lock motor could also be stiff. Try the manual locking lever on the inside of the door. Is that also stiff?
You can try spraying the latch mechanism with a generous amount of silicone spray lubricant. If that doesn't help, you'll have to remove the inner door skin and spray all of the linkage involved in locking the door.