Mine did the same thing on Saturday. One pass was slow then the next pass it only went about 1/3 of the way. I had to "help" it finish the pass so it wouldn't be stuck in the middle of the window. I have tried a couple of times since with the same results. At least I'm not alone.
Here's an update on this issue. The rear wiper stopped working entirely last week. Brought it in this morning to the dealer. Motor is completely seized. First time they've seen one on the WK2. They've ordered a new one. Going back on Thursday for that.
Easy fix as long as the problem was isolated in the wiper motor and not caused by something else causing the wiper motor to seize. Thanks for the update and keep us posted on the outcome of this potential "fix.'