There are a number of configs where max pressure tyres lead to fastest times - it seems fairly unrealistic that you can't keep pumping the tyres up. I suspect this problem exists because of too much difference in the rolling resistance of a high and low pressure tyre - but I have nothing factual to base that suspicion on, it just seems odd to pump tyre pressures up to such a level, at the expense of grip, but still go faster. This has occurred for me in the UFR and FOX specifically, but they are the only cars I've raced since I've really incorporated this into my setup development.

Scawen has told me that rolling resistance is worked out by the heat genereated as the tyre rolls and deforms. So perhaps then it is a case that the tyre walls are too soft, meaning they deform and heat more as the pressure is dropped, slowing the car?

that theory sounds pretty likely. I hadn't considered the possibility it was anything but an arbitrary calculation. I guess it could potentially come back to the 'too much tyre deformation' argument as well.