Why McLaren P1 Top-End Acceleration Is So Slow?

So the car and its "race" version (P1 GTR) have the same issue: the acceleration past 320 km/h is very poor. I just cannot compete with cars in the same class online.

But from the reputation point of view, the McLaren P1 is the member of the "holy trio" of hypercars (amont LaFerrari and Porsche 918). Top gear called it the most racecar raw feeling of the three. So it must be fast, right?

I just wonder why in this game the car was made so slow? Any particular reasons?