Example using Threaded Resource Loading of PackedScene
s in Godot 4 Aplha 9.
Includes simple transition effects.
This example requires Godot 4 alpha 9. It may or may not work with past or future versions of Godot 4. Godot Alpha 9 can be downloaded from: https://downloads.tuxfamily.org/godotengine/4.0/alpha9/ This download only contains an executable. Put the executable into your path and run it from the command-line. Or just "double-click" the exe, I guess.
After that, you only need to clone this project and point Godot at it. F5 will run the project from within the Godot Editor.
Have fun!