This is a case of fix this fault fast!
When the windows first start to get slow - and struggle to close - wind the window right down. Get some WD 40
or silicone spray
and generously coat the inside of the window channels - use the straw to get it al the way down inside the door. Spray the weatherstrip on the door too (the rubber lip that rubs against the outside of the glass).
Wipe off all excess spray and you should find that the window now works at its correct speed. If it's been allowed to struggle for too long, you may find that the regulator inside the door has begun to bend up - usually meaning replacement is necessary. This is why it's important to lubricate today!
You wouldn't believe how many times I've heard this, and so easy to fix! The customer opens the window when driving and is convinced of a fuel leak.
Firstly, please check underneath and make sure there are no physical signs of leakage. If you are worried for any reason, get the vehicle checked by an expert.
Now for the common problem - especially on Zafira's for some reason - was the seal on the petrol cap. If you unscrew the cap and look at the thin rubber seal, you might see that it is twisted/screwed up. It sticks on the filler neck and, as you tighten the cap, the seal is damaged beyond repair. It was always the green-coloured seals that were effected.
A new cap or seal is the simple fix and you will once again be breathing fresh air!