That system is complex. It can be a "find the problem on first try" or "keep going down the list till you are 5 levels deep" kind of thing.
The shop is going to instrument the car, have the shop manuals, follow the diagnostic codes flow chart and have to do that until they find the right cause.
I'd ask myself what the reputation of the shop was, what Porsche specific experience they have (I'd tell from the cars parked in front and in the service bays and if I'd see racer cars or torn down engines around), what Porsche specific diagnostic computers they use and (ask) what the price of not getting it right the first trip would be.
Is an hour and a half bad? No. Understand that, for the guaranteed price, their labor time to determine the cause could exceed my estimate, or not. They are taking risk. So are you. See the previous paragraph. A fix could also require parts and installation labor, or in diagnosing, the problem could get fixed with no parts. Mine was.
What are your alternatives? Go to someone else? Do it yourself?
My experience was for complex problems (and I only had one in 5 years) I was satisfied to go to the best shop by reputation, establish a good rapport and expect a fair price for well done work. I wanted them to want me to come back if I had another problem.
A peak at how complex ... based on my one problem.