Hi Skúli
I assume the wipers over time have been slower when they do the sweep..?
I think the problem is the axle's, where the wiper arms are fitted to the console. There are most likely some water in/around the axle, which causes corrosion around the axle.
If you still have warranty, let the dealer fix it.
If not, you can remove the wiperconsole and dismantle it, clean it, and grease it up again. I'm sure this will solve your problem.
If the wipers have not slowed down, the problem could be inside the wipermotor. I don't think that's something you can repair, I think that requires a new wipermotor.