One theory I've heard in that vein is glycogen - depleted muscles absorb a ton of glycogen (and water) and that is heavy. I am not a doctor, and not even a noctor, but it makes sense from what the smart folks on ST say. It also makes sense because completely contrary to what I expected today, I felt fan-freaking-tastic and rode like the wind. Low HR, power everywhere and any time I called down to the engine room.

And also, all the curves at speed showed just how underutilized the R2 is on the MUP; it loves being committed to hard corners at speed. Frankly it deserves a better rider.