Pop the inside door panel off and lube the mechanism as has already been suggested. Also check out all of the electrical connections, 12 volts DC isn't that high a voltage and therefore it wouldn't take an awful lot of corrosion on a dodgy connection to slow things down, I used to have a small Jap motorbike with 6v electrics so i know all about dodgy corroded electrics. Break and make all of the visible plugs a few times, also ensure the flexible wiring harness between the door pillar and the door isn't damaged, there is a lot of flexing and so a wire can easily get broken/damaged increasing its resistance.
You should be able to pick a replacement motor up from a local scrappy for peanuts, nowadays they often have these types of components on the shelf.