The rubber "window scraper" strips have something to do with it. There may be other things going on, but it/they might need replacing.

The inner ones have a velvet-type nap. When the nap gets worn off the window doesn't slide as nicely. It sort of goes bump-bump-bump as it goes down. Up is smoother. Both up = down are slower than with good nap due to increased friction. You can replace them if you pop off the door's inner panel. Front and rear aren't the same length, not sure which is longer. If you salvage parts from a junker, you can get the longer and cut it down about an inch to become the shorter one. As with door handles, rear ones usually see less wear and tear than front ones.

Not sure if the outer one has the nap - I don't think so. Anyway, if you do want to swap in an outer scraper, it comes off by bending out the metal trim's wrap-over clips at the very ends.
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Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).