I've had this happen more than once, its the window motor (at least I'm assuming you have power windows and your arms aren't just going slow).
Could also be the rollers, if they go bad they will slowly kill the motor too. If you're going to replace the motor, I'd say lube the rollers up with a little white lithium grease while you're in there. Helped mine a bit, and a few of my rollers are shot.
