Of all the ways I've tried to make windows and window motors work better together, the one thing that I found helps most is good lube on the window channels. There's one product that I really like. It's called Jig-A-Loo.
Every time I have a door panel off I treat the window channels. This really helps speed up the windows and probably take quite a load off those old motors. I also take the time to lube the channels after new window motor replacement. As many of you probably already know, it's become hard to source good quality window motors and some of the imported replacement motors need all the help they can get.
I've tried many of the lubes out there. Many either leave a white residue on the glass and rubber, or in a few days they loose their effectiveness. So far, Jig-a-Loo is my favorite. Look for it in the orange can. You should be able to treat between 10-15 slow windows per can, so, it's very economical to use. Plus, you'll find other good uses for this great product. See: http://www.jigaloo.com
Here's some Jig-a-Loo videos courtesy of YouTube...