I still think it may be caused by the fact that the plane was too heavy to maintain 30,000 feet (where the air is thinner.) If you look at Flight Radar 24 you'll see airliners leaving the west coast for Asia are all at about 30,000 feet at 100 miles out, then they begin to climb, and as they approach their destinations, they are at 38 or even 40 thousand feet. Inbound traffic to the west coast is at FL 38 or so. I'm sure it's the same on the East coast for traffic to Europe.

Asobo probably has this effect built into their model.