Found this question on AskGodot. https://ask.godotengine.org/60353/how-to-wait-the-whole-scene-load-to-execute
I didn't quite understand what the answer was on about. A reddit thread (linked to using Await)[https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_basics.html#awaiting-for-signals-or-coroutines], but offered no further details
I've also found this tutorial for Background Loading, maybe it can help you? https://docs.huihoo.com/godotengine/godot-docs/godot/tutorials/engine/background_loading.html
As for the significant difference in load time on linux, that's really head scratching. Similar enough graphics cards on both? What render are you using? OpenGL, Vulkan?