take it apart and re grease the spindles.ive heard that the motor/mechanism from late mk1 cabriolets are better but havent tried it myself.
Do the above first. If they are still slow after that then check the motor isn't faulty (they can burn out after doing too much work with stuck mechs. And then check earthing points.
On my Mk2's I've noticed the wiper arms get stripped at the connection screws and they start going very slowly. I replaced the arms and everything was fine. Good luck!
