Mine are also slow but I won't replace the motors - heres a little insight.
When the windows get slow its usually not the motors going bad, its either the alignment gets off, the lubrication used on them is worn completely dry and has become tacky, some rust could be to blame as well from poor lubrication, or the brushes in the motors need replaced.
Those motors have used brushes, and thats why they wear out. DC motors are extremely simple and easy to clean up and replace the brushes. You'd probably spend more time finding the brushes for them than actually changing them from the motor.
When a dremel wears out, I hope you don't just go buy a new one. If so please send me your old "worn out" dremels, I'll pay for the shipping and the 4 dollar brushes.
EDIT:
Since you replaced the motors, what kind of motor is used?(serial number bar code ets) I need to find out aside from taking my door apart twice- once to get the motor number and once to replace the brushes.