> In reality, it seems that people have just settled on a response time that's acceptable and devs try to stay within that window in most cases.

More specifically, every time CPUs or browsers get better, publishers look at their metrics and go "Oh! PLT at 95th percentile is down! Great, now we can add more trackers while still hitting our perf target!" and so the gain is immediately gobbled up -- we're in a Red Queen Race and websites will never get faster. I cannot wait for the adtech ecosystem to collapse, what a godawful waste of resources this is.