Since the earth is turning to the east, a plane flying westbound gets there faster because the earth is turning underneath it. It's just like if you're on a train going 60 mph and you toss a ball up in the air, the ball will shoot out of your hand at 60 mph and crash into the wall before you know it. That's why you should always use the forward lavatories when flying east and the aft ones when flying west, and never flush when you're at an intersection of airways.