At first glance, higher altitudes equal lower temperatures, which would likely slow down cellular metabolism. However, while backpacking in New Mexico in ~90 degree weather, I experienced the same thing -- the flies were slow enough I was able to catch them easily in my hand, as well as with chopsticks.
Is this related to air density? Why are these flies so much slower to react and move about in the mountains.
Also, does this question even belong here?