You know the parallax effect. Maybe you do not know that you know the effect. In case you are not convinced, imagine the following. There is a background, a midground and a foreground. If you move, the foreground moves before your eyes fastest and the perceived movement gets slower the more it is set in the background.
I built such a picture in powerpoint. The mountains are in the background, the trees are in the midground and the word is in the foreground.
I also added some animation. I used the animation path to get the elements to move and set the starting time to the same point. I started with the settings you see in the picture. This was too fast. A little movement is enough. The midground seemed to look OK with a maximum of double the way compared to the background. The foreground can be twice to three times the speed.
There is one thing which I deem important: In order to create a realistic scenery the way the speeds behave to each other have to be the same as the distances of the different planes (back-, middle- and foreground) to the vanishing point. If the area is very wide, you might also keep the background still and just move the mid- and foreground. If your scenery is on the inside consider moving the background in the different direction and keeping the midground where it is.
Feel free to download the powerpoint and use it to play with the settings. If you need help, let me know.