I don't have an answer. I have rebuilt the mono wiper on my 300TE, and was amazed at how little grease remained inside the head, and that it was dried out and hard. I rec that you dissemble one of the wiper heads, clean and relube, and see if that helps your sluggish problem. It may be that at high setting the momentum of the wiper overcomes the resistance of the dried out lube, but at low setting, the resistance of the dried out lube bogs things down. There may also be a worn area along the track inside the head that bogs things down. The mono wiper from my TE would not retract, and inside the head, the toothed track was totally worn done, literally a pile of plastic dust!

Just an educated guess after dissembling two of these mono wiper mechanisms last weekend.

See the thread below about how th grease the 300E wiper gearbox.