Fun video to watch, thanks for that!
I can kind of see both sides to the equation there. Infineon is not a horsepower track; having driven a R8 4.2 around it in anger I can say that a big mid-engined wedge with significant power can feel a little claustrophobic around an undulating twisty course like that. In contrast to say, Laguna with its longer straights and big sweeping turns, Infineon rewards precise braking and apex clipping which can be tough for a new driver in a big cruise missle like the McLaren. I'm not surprised he was uber-conservative with the braking and cornering speeds.
My guess is that he was a bit worn out mentally after several laps, easy to do when you're maybe not used to track days.
Obvious the OP knows what he's doing
a track tuned 135i is perfect for that place as well!