This is a good point, the A/C draws it's power at all RPMS. I would think it would steal more power as you drive it faster.
The other draw is electrical. When the A/C comes on, the electric fans are usually commanded on, as well as the blower motor. So you put more load on the alternator as well.
Finally, add that you use the A/C when it's hot and your engine makes less power in hot weather and you really feel the loss of the A/C
FWIW,
TB
