Once removed, inspect the regulator. The primary metal structure of the regulator should be relatively flat/straight. Inspect it for bends. Inspect the teeth on the large regulator gear and on the small motor gear for uneven wear or broken teeth.

On this particular regulator used in this example you can see it is very obviously bent away from the motor.

This is causing the large gear to rub against the motor body in two places. This explains the slow movement of the window even after greasing the felt slides on the window rails; the friction of the gear rubbing on the motor housing is stressing the motor.

The teeth on the gears and motor showed some wear but still appeared serviceable and none of the teeth were broken. If this was not the case the process would need to stop here while a good used regulator was sourced.