There are the physics change on a side, and the changes of some suspension and downforce values in the other side. It is the later that result in slower lap times.
As an example, they totally destroyed again Gr.1 cars, or at least all of the Hypercars which are now 20km/h slower in the straight lines and 5 seconds a lap slower at Le Mans. They just can't help themselves from twisting reality to shoehorn these cars into absurd performance boxes. That is one of the main reasons why I will always prefer the addition of road cars over race cars in GT7.
Changes made to the physics are really good as a controller player. The downside being all my setups are to be thrown away, as are all my custom race grids....
The Caterham Museum is nice, even though it lacks many photos from the early decades. Their renewed activity in 2013 is quite rich in informations, their closer ties with Alpine are even mentioned
(they both went back to Le Mans around the same time : 2013 and 2014). Yet, surprisingly, they overlooked this :
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This 2014 "Caterham" LMP2 car was added late in Gran Turismo 6 through an update, but with its 2013 Greaves Motorsport livery.