Have you tried thoroughly cleaning the seat belt fabric itself? I don't think what you are describing is necessarily an mk3 or even vw specific issue. Over years of putting your hands all over the seat belt and the sweat / dirt from your clothes make the belt fabric more stiff / dense and it doesn't retract as smoothly because it takes more effort for the material to bend and roll up.
To clean the material properly you need to let it soak for a little bit, it won't work well if you just try spraying some all purpuse cleaner on there and wiping it off.
Pull the seat belt all the way out till it stops, then use something like a clothing pin or a clamp to hold the seat belt from retracting back. In a bucket or any appropriately large container, put warm water and your choice of cleaner, that could be dish soap, APC, laundry detergent.
Submerge as much seat belt material as possible in the bucket. Move it around and agitate a little. You can even use a tooth brush or soft bristle brush to scrub it. Once you feel it is clean enough, take it out and dry with a towel/hairdryer/ air dry.
This should leave the material of the seat belt nice and clean and as a result it should retract much smoother / faster as it should be nice and flexible to fold & roll up.
If after a proper cleaning you still find the seat belt is abnormally slow to retract it could be some other cause but start with the cleaning and go from there.