We target <3s to usable on lighthouse mobile, ideally less but I don’t design the sites so only so much I can do about large above fold images etc. the truth is react isn’t that bad compared to latency or other issues on the kind of use case you’re describing

I should clarify - this is headless international ecommerce, so that 3s includes localisation, currency, inventory, cart, video, tracking scripts etc. If you'd like 200ms to live on a static site I can do that too, it's just a bit different.