Thank you for giving it a try, fellow godotner !
Strange bug of yours :). I can run it on Firefox with 300 people without problem on my computer while I only have a low-end integrated GPU and 16Gb of RAM... and in the build I put only 100 people, just in case... I'll try to make a proper version next week-end.
Edit: and the HTML build works (slowly) on my old android smartphone too. Apple and Godot...