Some neat info on the performance side of things, Fleet is using Skija[1] and Compose[2] as a rendering engine and UI framework. Skija is a java wrapper for Skia and still does not support the full API. Compose is still relatively new and has not had nearly the amount of man hours devoted to it as Swing. This is just a technical preview, I'm sure there will be plenty of optimizations to come. I think it's pretty exciting to see Skija being used for a mainstream project!
[1] https://github.com/HumbleUI/Skija
[2] https://github.com/JetBrains/compose-jb