I agree that I don’t think this is about simultaneous users per se. When I run workshops I routinely get an entire class to run a study at the same time and it never leads to an issue.
I expect it is the complexity. I wonder, especially, if you have lots of things changing on every screen refresh? See if you can reduce any of those rapid changes. We’ve given people a lot of flexibility by allowing all stimuli to change on every screen refresh (as we do in Python) but I think at the moment this is less robust in JS on older machines