When I fly my Mavic in southern US (Texas) it's very responsive and flys fast.   Sport mode is even better.    However, flying in northern US (Montana), its very slow and not as responsive.   Does the thinner air reduce the Mavic's speed that much?   Top speed in Texas was 40mph, top speed in Montana is about 15mph.  

Normal?

Never heard of that. I'd say no.

Did you recently perform a firmware upgrade? I find after the firmware upgrades I have to enable multi flight modes again as it disables it by default. This will restrict the speed even if you put your drone into sports mode until you enable multi-flight.  

I wouldn't think there would be such a difference are you sure it was in sport mode in Montana?

Thermal winds? Just kidding. How high were you flying? Wind speed on the ground can be different than in the air maybe this affected it?

Higher altitude, higher temperatures, and higher humidity all cause an airfoil to have less lift and therefore decreased performance. This is one of the test questions which is important in the "Part 107" airman certification test for Unmanned Airial Systems (Drones) that the FAA gives.