That's interesting, I'm the opposite! Physical exertion is more likely to hit quicker for me. Exception is after an exam, which involves the physical strain of getting there plus the one to two hours of mental strain doing the exam. After I finally get home, straight to bed to pass out. That's the only time cognitive exertion really crashes me, I guess because I usually stop cognitive exertion when it starts becoming too hard. I study in short bursts to reduce the effects. Physical exertion is harder for me to avoid crashes with, harder for me to judge I think. POTS is probably a factor in that, because it makes me feel horrible when I do things regardless of my CFS, so makes it harder to judge the effects of exertion on PEM.