Clean the belt?
Maybe.
Lube it?
Possibly.
The problem is the return spring is fagged out ( Queefed? ) and not
pulling the reel back as fast with the belt material wound it any more.
The spring is unsprung.
Cleaning the belt is a good idea, so you don't get your "Sunday go to
meetin' " suit dirty.
I doubt that cleaning it will provide any worthwhile results.
Buy New.

RC

<<Subject: Re: Slow retractable seatbelts... fix?
Good theory. I'm in agreement that it's the belt itself, but I'm not
jumping on the "shut the belt in the door" solution. You need to clean
the entire length of the belt that retracts into the housing for it to
be effective. It also wouldn't hurt to lubricate to "reel" as well.
Removing the entire assembly is the only practical way to clean it
correctly without dampening the inside of your Vanagon.
Jason