The lag is caused by the first instance of GPUParticles3D, which seems to only happen in a web build.
Related git issue: https://github.com/godotengine/godot/issues/87891
I have fixed the issue by instantiating them on a loading screen. The new build needs to be uploaded by Toxii, though.
Seems like we didn't release binaries, so the third parties file isn't included. I'll include them here;
Lava shader: https://godotshaders.com/shader/stylized-lava/
Portal shader: https://godotshaders.com/shader/snells-window/
Air physics from SimpleFPSController: https://github.com/majikayogames/SimpleFPSController/
