If your vehicle is showing windshield wipers move very sluggishly, here are the most likely causes and the parts you'll need to fix each one.
Over time, the grease inside the wiper linkage pivot joints dries out, or water intrudes and causes rust. This creates severe mechanical drag that the motor struggles to overcome.
The internal brushes of the wiper motor wear down, or the internal gear grease hardens, causing the motor to lose torque and operate sluggishly.
Corrosion at the wiper motor's electrical connector or ground point reduces the voltage reaching the motor, resulting in slow operation.
If the wiper linkage or motor was recently replaced, the linkage arms may be misaligned, rubbing against the cowl, or the pivot nuts may be overtightened, causing mechanical binding.
If the vehicle's charging system is failing, the low voltage will cause all electrical motors (including the wipers) to run slowly. This is usually accompanied by dim lights and starting issues.