re: lag I suspect it's from a long-standing issue with particle/shader compiling, most apparent in web builds.
I think for games with lots of particles, it calls for addressing it with some of the workarounds suggested in these threads/videos.
(I'm trying to track down if there is a solution implemented into Godot Engine, but I don't think there is yet)
https://github.com/godotengine/godot/issues/29973
https://travismaynard.com/writing/caching-particle-materials-in-godot