I checked it, the verson I uploaded has thread support enabled and only runs with this experimental shared array buffers option.
So this doesn't seem to be the issue. I got even weirder, when I tried it on different machines in the browser, a laptop running linux where it runs smoothly, and a windows pc, encountering these performance issues.
I wonder, if I just went too high with shader complexity / geometry (polycount), particle effects and / or post processing.