S80 1999 Fuel consumption around 15 lt/100km and low power

[Marke]

Hi Guys,
I'm driving my wifes car at the moment and find there's not very much power (2.9ltr 6cyl) engine and the fuel consumption is around 15lts/100km in stop start traffic over a 15km trip.
I put the car into a service shop and he confirmed there were no fualts and all sensors are doing as they should.
The car has high octane fuel always.
I don't drive this car often so maybe this is how it is so I thought I'd ask the question.
Does anyone think the car has a problem or do they just use a lot of fuel and have no power?
I was wondering if the catatytic converter blocks up or something?
Regards

[Marke]

Just a bit of a follow up.
I took the car to the airport and reset the trip computer as I entered the freeway.
So an open and easy run over 1/2hour at 100km/hr gave 8.3 ltr/100km, so I thinks it's just about to spec.
Must just be a heavy car around town!

Any thoughts?

Regards Marke.