Since the jet streams flow from west to east, they make one leg of the journey much faster (when flying with the stream) and one slower (against the stream). Imagine going downstream or upstream a river, or how it feels when you are cycling against the wind, as opposed to when you have it at your back.

Flights are faster when traveling eastward (e.g., New York to London) primarily due to the Jet Stream, which are narrow bands of extremely strong winds in the upper atmosphere that blow from West to East. These winds are caused by the Earth's rotation and the atmospheric heating from the sun. When a plane flies eastward, it hitches a ride on this "tailwinds," which can boost the aircraft's ground speed by 100 mph or more without requiring extra engine power. Conversely, when flying westward, the plane faces "headwinds" and must fight against the air current, which increases fuel consumption and flight time. A common misconception is that the Earth's rotation itself "moves" the destination toward the plane, but because the atmosphere moves with the Earth, the effect is entirely aerodynamic. Pilots and airline dispatchers use sophisticated weather models to "catch" the strongest part of the jet stream for eastbound trips and "avoid" it for westbound ones, which is why a flight from Europe to the U.S. is almost always an hour or two longer than the return leg.