The Earth rotates from West to East, so that means the Earth does a bit of free work to make the trip shorter going West than going East .Of course, winds tend also to approach us from the West so we will get a resistive headwind flying West, and a supportive tailwind flying East on average. The boost diminishes with increasing latitude from the equator because the rotation velocity decreases from 1100 miles/hour at the equator to zero at the poles, by the cosine of the latitude. At 60 degrees latitude, the speed is only 550 miles per hour, but when you factor in headwinds and tail winds, you do not get the full benefit of this as an added or subtracted boost to your air speed.