Altitude does not have any effect on the ability of an air mattress to remain air tight (unless you drop your knife on them or burn with stove or have anywhere near a fire-pit). Therefore, the only altitude-related inflation issues you'd encounter would involve becoming light-headed faster as you blow the thing up as altitude increases. Acclimatization and quitting smoking really helps. This is a great mattress in any conditions at any altitude.

The density of air at higher altitudes is less than the density at sea level because the air pressure is much greater at sea level (by up to 20%). Because of this you need more air to fill a sleeping pad at higher altitudes. But that holds true for any pad. The Neoair has pretty standard dimensions, so it isn't any harder to blow this up than any similar pad at the same altitude.

Yea, I have it. It takes more to inflate than the other thermarests because there is no foam on the inside for it to self-inflate. It is also 2.5 inches thick. I get a little light headed after a 20 mile day, but I wouldn't trade that in for the world, best pad I've had yet!

The only reason it would appear to take a lot to inflate or take longer to inflate would be the effect of altitude on the person using the mattress. There is no physical property of the mattress that would cause the mattress to use more air or take longer to inflate at higher altitudes.