After you get the door handle off (without removing door panel) as Peter said, you should be able to tell if the resistance comes from the tumbler assy or inside the door at the latch.
The tumbler is relatively easy to diassemble and should be if the resistance is there. When the tumbler has been released from the handle (by removing the actuating lever and the clip, be sure and remove with the key installed. This keeps any free tumblers from springing or falling out (saves having to recode the tumbler on reassembly). In your case if the bind is the tumblers they will probably come out near frozen. each will have to be pried with a pick, out of the body. All should be cleaned, including the body and reassembled with light lubrication.
Bosch Master, ASE Master, L1
33 years MB technician