Thanks for your reply.
Just for context: The projects I'm trying to migrate from SceneKit to RealityKit are 3D games (non AR, non spatial, just classic top-down 3D games).
I was using the 'content scale factor' property of SCNView to increase the rendering performance on complex scenes by limiting the content scale to 2x on devices that render by default at 3x and also dialing it down to 1.5x on old devices in order to maintain 60 fps, which is my target.
I found that turning down the scale factor was a simple and easy way to increase rendering performance without compromising rendering quality too much, especially on the small iPhone screen, where the 2x - 3x difference is hardly noticeable when playing a 3D game. Going from 2x to 1.5x on older iPads was also the key to reach 60fps.
Obviously, I could just reduce the scene complexity, draw calls, post-processing, etc. on older hardware in order to achieve the same result, but I find more convenient to turn down the scale factor.
Anyway, it seems it can't be done in SwiftUI with RealityView, so I may just continue using AppKit and UIKit.