Watch your language...
On the topic at hand, going fine pitch/high RPM while flying in low-density air decreases performance after a certain point. The air is "thin" so you need to coarsen the pitch (lower RPMs) to get a sufficient bite of air, otherwise the prop just spins around really fast without pushing the needed amount of airflow.
It's modeled the same way in 109s by the way (you get better performance if you drop your RPMs a bit at higher altitudes) and it's one of the first things i noticed and really liked in the sim.
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