My apologies, but I made a whoopsie with the GT-R.
I gave you an improperly updated version of the car. The proper one has massively different physics/FFB, and drives more like you'd expect it to, which = faster laps.
You can fix this without downloading anything by doing the following in Content Manager:
Navigate to the JT5 2016 Shiro in your car selection menu and right click it (it's the first one in the list), then click 'open car in content tab' and close the car select window.
At the bottom of the screen, press the button labelled 'pack data' and let it overwrite the old one.
Done.