Cause for concern?
 

2006 XK8, 95000 miles, second owner. Noticed of late the drivers window raises & lowers slower than the passanger side window. Is this the harbinger of failure? Time diferance is about a second.

I've never timed the difference when it comes to raising and lowering the power windows on either of my Jaguars. I'm just very happy if they go up and down when I press the button!

However, it does takes much longer for the power window on the driver's side of my Ford truck to go up and down. It's been like that for the past eleven years, so I would have to answer your question by saying no, this is not the harbinger of failure. Enjoy the drive. :)

As I've found when working on door glasses, it takes a surprising effort to raise a door glass. If the Guide Rail (arrowed in red) is dirty, this can increases the resistance considerably and may explain why your driver's door is slower.

Attachment 123202

There is adjustment on the Guide Rail (circled in blue). These have been known to work loose.

Graham