The system works way better when relays are wired into the wiring so that direct power goes right to the motor. From factory all the power flows through the switch which creates a drop in the flow of juice.
There are plenty of relay right ups for the power window system that will hide out of the way, look stock, but make a huge difference with the amount of current being sent to the window motor.
As a test, yes, hook up direct power to the motor and see what happens. This will be just a quick check if the motor is the issue and not possibly something else binding the works!