Suspect it is partly the worm drive in the motor, all of my GM cars have slow power windows but speed is essentially the same up or down. In fact on one car the window regulator spring was missing entirely and the speed difference was only noticable if you rolled both fronts up at the same time - the one without would get about an inch behind the other at the top.

BTW I had terrible milage with original Goodyear GT +4s, like 20k before baldinis. Switched to Michelin X-Ones on all drivers and on the TranSport after 45,000 miles am down to 6/32s. Think I may go a size larger than the stock 205x70x15s on the next set but 6" rims are limiting factor.

Bring windows all the way down.
Spray some silicone on a rag, wedge it between the felt tracks and spread up
and down.
Do this for all windows about twice a year. Especially after winter when dirt and sand
lodges in the back track.
You will see a noticable improvement in speed.