Ahhh..the magic of flight.

KMHT FF has it correct. The air is so "thin" at higher altitudes that it takes much more power, speed, lift, etc. to accomplish a given task than at lower altitudes.

Consider this...the average jet has a "Never Exceed" speed of around 400 kts. If that's the case, how come they often travel around 500 kts or more at 30,000 ft? The reason is the thinner air allows them to go faster. Ironically, the airspeed indicator will still register SPEED THROUGH THE AIR of about 300-350 kts. but the SPEED OVER THE GROUND will be 500+ kts!

My numbers may be off but I think you get the point.