IME one cause of slow wiper movement is if one of the magnets pop loose inside the case. A clue that this may have happened is if the motor is louder than it was before.
And if a magnet pops loose it can (magnetically) stick to and drag on the armature. If both magnets come loose the armature won't turn at all, IME.
Here's a link with photos and discussion:
Thought I'd post up what I found when I pulled the original windshield wiper motor. It sometimes would run slower and was making a sorta growling noise which was more noticeable after I had removed the wiper arms. With the motor apart I found that the magnet on the bottom side of the case had...
forum.ih8mud.com
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Below are a couple of photos from two different FZJ80L front wiper motors showing what happens when the inside of the case rusts which can cause one or both of the magnets to pop off.
Look closely at the top photo (click twice to magnify): one of the magnets has come loose and slipped down inside the case. Note also the grind marks from the armature rubbing on one magnet. The rust is visible at the bottom of the case.
View attachment 3141359
The next photo shows a motor that would just get hot, no movement of the wipers at all. When the case was pulled off both magnets came out with the armature:
Again, note the rust on the inside of the case where the magnets
used to be attached (click photo twice to magnify):
View attachment 3141360
Unfortunately I think this may be the fate of most (?all ) of our front wiper motors as the inside of the case does not appear to have any sort of rust-resistant plating, or if it did originally it wasn't enough to prevent rust.
FWIW