I bought Delco motors from Rockauto for about $25 each for the DD. The pass motor was original, the driver side gets used a ton and was replaced about 10 years ago. Both were very slow. I removed the door panels to replace the motors, and when I powered them up to position them for removal, they hauled ass. Normal speed. Both sides. WTF?, all I did was remove the door panels and they worked great.
Turns out the inner rubbers were so stiff from age they were binding the glass up. I "broke their back" with my fingers, reinstalled the panels and they've been fine for over a year.
It was the stiff rubber.
New motors are cheap, and not too fussy to replace if you know the trick to drill the rivets easily. (Blast the center pin out with a punch before you begin to drill. Use a 5/16" bit to drill the head off). I can do both sides in about an hour if I have everything on hand like door panel clips, replacement 1/4-20 bolts and lock nuts to install the regulators instead of rivets, etc.
But be sure to check the belt line rubber.