It is said that air 3 Gen has 4GB Ram. So does my Ipad Pro 10,5".
Ram is the culprit.
You should be able to do minimum same that I did here
or check my complete thread here.
In short: You’ll have a lot of fun!!!
Maybe you need to use very nice decmation function a bit more often
3million polygons are the comfy border. Even though you can go up to 6 or more depending on the scene.