Should be "optimized" exactly the same. Which iPhone? Which browser? (Do you even get a choice on an iPhone? I don't know.) Have you tried previewing that particular game over LAN on other devices?
It was an iphone 3s?
I know, its old, but I have updated it since, so maybe it will perform better now its been updated to the latest OS? (Using a relatives iphone, I don't own one myself - don't think they are very tech savvy, haha)