If you "pre-warm" everything at once the browser might freeze or crash due to lack of memory if you have a lot of particles and VFX made. I would do something Rust does when entering a game, theres an loading screen that loads each prefab/vfx/etc. separately in the background with a counter (progress bar). Load them in separate thread so that the game doesn't freeze.
Implementing this should be fairly easy, start by accessing the folder(s) all .tscn files are located, this could be like a dictionary with paths and iterate over them instanciating each one and incrementing the progress after it exits the tree.
You can hide all of this under some UI.
